Process and system for producing commercial quality carbon dioxide from recausticizing process calcium carbonates

ABSTRACT

The invention features methods and systems for recovering carbon dioxide, for producing commercial quality carbon dioxide (CO 2 ) of 90% to +99% purity using, wet calcium carbonate lime mud produced in a recausticizing process that also produces caustic soda, for instance, Kraft paper pulp mill lime mud (a.k.a., “lime mud”) as a feedstock to a multi-stage lime mud calcination process. This process may be fueled with low, or negative cost “carbon-neutral” fuels such as waste water treatment plant (WWTP) sludge, biomass, precipitated lignins, coal, or other low cost solid fuels. High reactivity, high-quality calcined lime mud (a.k.a. re-burned lime, or calcine), required in the Kraft paper pulp mill&#39;s recausticizing process is also produced, and superheated high pressure steam and hot boiler feed-water is generated and exported to the mill&#39;s steam distribution and generation system as well as hot process water for use in the mill&#39;s manufacturing operation. The system for calcining calcium carbonate lime mud produced from a recausticizing manufacturing operation and converting it to calcined lime mud and CO 2  comprises a calciner and a combustor linked by a moving media heat transfer (MMHT) system or apparatus. The MMHT system or apparatus thermally links separate fluid bed combustion (exothermic) and calcination (endothermic) stages with a solid particulate media. The system further comprises a flash dryer or spray dryer that utilizes exhausted enthalpy from the calcination stage.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of U.S. Ser. No.11/895,440, filed Aug. 24, 2007, which is a non-provisional applicationclaiming the priority of provisional application Ser. No. 60/840,319,filed Aug. 25, 2006, the disclosures of which are incorporated byreference herein in their entireties. Applicants claim the benefits ofapplication Ser. No. 60/840,319 under 35 U.S.C. §119(e).

FIELD OF THE INVENTION

The present invention provides methods and systems for producing highquality carbon dioxide (CO₂) of, for instance, 90% to +99% purity.Further, the present invention provides an improved process forcalcining calcium carbonates or “lime mud” as produced in, for instance,a Kraft paper pulp mill or other alkali-based manufacturing operationsthat utilize a recausticizing process to produce caustic soda. Such limemud may be converted to commercial grade CO₂ and high-quality calcinedlime mud (re-burned lime, or calcine), using only low, or negative cost“carbon-neutral” fuels such as biomass and biomass derived negative costwaste-water treatment plant (WWTP) sludge, non-condensable waste millgas (NCG), or low cost solid fuels such as coal, petroleum coke, etc.with the co-production of superheated high-pressure steam. The presentinvention also applies to recausticizing process calcium carbonates asproduced in the aluminum industry or by the capture of CO₂ from air.

BACKGROUND OF INVENTION

The major market regions for liquid CO₂ are the United States, WesternEurope, and Japan with the United States being the largest consumer. Themajor CO₂ end-use is for food processing and carbonated beverageproduction.

CO₂ is usually recovered as a byproduct from bioethanol production andcatalytic steam reformation of natural gas followed by the water-shiftreaction to produce anhydrous ammonia. In the United States, there isincreasing CO₂ availability from bioethanol production due to the needfor “green” transport fuels.

New bioethanol CO₂ sources are increasingly located west of theMississippi River due to corn feedstock availability, whereassignificant liquid CO₂ demand is in the densely populated Northeastern,Southeastern, and Southern states.

Most of the Kraft paper pulp mills in the United States are located inproximity to regions of high liquid CO₂ demand. It would be beneficialif new CO₂ producing sources could be created within these high needregions.

Further, many global paper mills have Precipitated Calcium Carbonate(PCC) “satellite plants” that supply this important paper filler topapermakers. PCC production requires carbonating industrial slaked limeusing CO₂ contained in adjacent pulp mill lime mud calciner off-gases orpaper mill boiler stack gases. There are supply reliability, quality,and cost issues associated with this approach such that a more reliable,higher quality and consistent CO₂ feedstock source would be attractive.

Also, there is a growing need for CO₂ within Kraft paper pulp mills toprecipitate organic lignins from aqueous “black liquor” fuel streamsnormally supplied to the mill's chemical recovery boiler. Thisde-bottlenecks the boiler while creating a valuable, new carbon-neutralbiomass derived fuel that can displace fossil fuels.

The Kraft pulp and paper industry is also a major energy consumer, withthe majority of that need being met by low cost, carbon-neutral, biomassand biomass related fuels. The conventional lime mud calcination processhas, however, not easily been converted to biomass fuels and remains aconspicuous consumer of high cost, global warming fossil fuels. In theUnited States, there are many Kraft paper pulp mills rated atapproximately 1000 air dried tons per day (adtpd) bleached pulpproduction with each requiring about 320 tpd of calcined lime mudproduction at an annual natural gas and oil consumption of approximately625 billion Btus. At 2014 energy prices this is approximately US $2.5million per year per 1000 adtpd Kraft pulp mill.

It would also be useful to regenerate concentrated CO₂ from more diluteCO₂ sources as the need for large scale “greenhouse-gas” capture andsequestration projects develops. One capture process(www.carbonengineering.com) utilizes sodium or potassium hydroxides tocapture dilute CO₂ from ambient air and could benefit from theregeneration of concentrated CO₂ from recausticizing process calciumcarbonate.

The Bayer process that produces “pot-line grade” alumina feedstock frombauxite ore for aluminum metal production consumes significant amountsof caustic soda and rejects large amounts of caustic contaminated gangueknown as “red mud”. Technology developed by Alcoa utilizes CO₂ toneutralize this caustic contaminant and reduce land fill pollution couldbenefit from the present invention.

In the Kraft paper pulping process, cellulosic wood chips are mixed withaqueous cooking liquor (a.k.a. “white liquor”) composed primarily ofsodium hydroxide (NaOH), sodium sulfide (Na₂S), sodium carbonate(Na₂CO₃) and sodium sulfite (Na₂SO₃). This mixing occurs in a “digester”vessel at a temperature and pressure satisfactory to separate thecellulosic fiber from the natural lignins that bind such fibers.

The liberated fiber is separated from the resultant “black liquor” andis subsequently washed, bleached (or remains unbleached) and iseventually transformed into numerous paper grades.

The separated black liquor contains, aside from the original whiteliquor chemicals, lignins and other organic matter that previously boundthe cellulosic fiber. In order to recover and recycle these costlypulping chemicals, as well as produce valuable steam and power from thecontained organic lignins, the black liquor is concentrated inmultiple-effect evaporators and delivered as a concentrated fuel to achemical recovery boiler.

This chemical recovery boiler combusts the organics under uniqueoxidizing/reducing conditions to produce both superheated high-pressuresteam and a molten inorganic ash (“smelt”) consisting primarily of Na₂Sand Na₂CO₃. The co-produced high-pressure steam is subsequentlyexhausted via a steam turbine/generator to produce mill power andlow-pressure process steams.

The smelt is drained from the chemical recovery boiler and quenched inwater to create “green liquor.” This green liquor is subsequentlyclarified and filtered to remove insoluble impurities whereupon it isdelivered to the “slakers” to initiate conversion of the dissolvedNa₂CO₃ into NaOH required for the white liquor. This slaking processutilizes calcium oxide CaO (a.k.a. calcined lime mud, or re-burned lime,or calcine) to convert Na₂CO₃ into NaOH via the following twoconsecutive reactions:

CaO_((s))+H₂O→Ca(OH)_(2(s))   1)

Na₂CO_(3(aq))+Ca(OH)_(2(s))→2NaOH_((aq))+CaCO_(3(s))   2)

The slaker product slurry, consisting of all the chemicals involved inreactions 1 and 2, is fed to subsequent recausticizers where reaction 2nearly proceeds to completion with some residual Na₂CO₃ remaining in thewhite liquor. The resultant white liquor mix of NaOH, Na₂S, Na₂CO_(3,)and Na₂SO₃ is physically separated from the precipitated calciumcarbonate (CaCO₃), or “lime mud” and recycled to the digester toinitiate the pulping process.

The lime mud is further water washed and filtered to recover as muchwhite liquor as economically possible before being fed to a rotary kilncalciner which converts the lime mud into calcined lime mud, (CaO andimpurities) for recycle to the slakers. During the washing/filteringprocess, trace amounts of residual Na₂S are air oxidized into morestable sodium thiosulfate (Na₂S₂O₃) to reduce noxious total reducedsulfur (TRS) compounds which can be created in and emitted by the rotarykiln.

The highly endothermic lime mud calcination reaction typically occurs ina rotary kiln, although fluidized bed calciners have also been utilized.Use of an external lime mud flash drying (LMD) process, when combinedwith the rotary kiln, creates the current “state-of-the-art” optimizedenergy consuming lime mud calcination process.

The first fluidized bed (“FluoSolids”) lime mud calcination process wascommercially introduced in 1963. It initially gave significantcompetition to rotary kilns due to its relatively lower fuelconsumption, higher product quality, and compactness. It fell intodisuse, however, as rotary kiln/LMD technology re-captured the fueleconomy lead and FluoSolids installations experienced operability issuesand an inability to economically operate at the high unit capacitiesrequired by a “world-class” Kraft paper pulp mill.

The kiln's primary endothermic (T_(R)=25° C.) calcination reaction is:

CaCO_(3(s))→CaO_((s))+CO_(2(g)); ΔH_(IX)=42.5 Kcal/gm mole (891,764Kcal/mt CaO)   3)

The rotary kiln calcines the mud between 1000° C. (1832° F.) and 1200°C. (2192° F.) and at CO₂ partial pressures well below the atmosphericpressure equilibrium concentration for these temperatures. This producesa calcined lime mud having the best physiochemical properties suitablefor subsequent slaking and efficient recausticizing.

Due to the high calcination temperatures, and so to not contaminateand/or upset the recausticizing process with inorganic impurities,either high-cost oil and/or natural gas fuels are utilized as kiln fuel.Low-cost solid fuels such as biomass, waste water treatment plant (WWTP)sludge, coal etc. are typically not used due to their contaminating ashcontent. WWTP sludge and biomass have the added penalty of loweradiabatic flame temperature due to the high water content.

Accordingly, while many energy-intensive pulp mill operations haveconverted to low-cost waste and biomass fuels for energy productionsince the 1970s, the rotary kiln remains a conspicuous consumer ofpremium liquid and gaseous fuels. While advances have been made toreduce this premium fuel consumption, it still remains between 1.4 (withLMD) and 1.7 million Kcal/metric ton calcined lime mud dependent oninitial mud moisture content, calciner capacity, fuel type, product limeavailability, and installed energy conservation features.

Due to various limitations, attaining future significant fossil fuelconsumption and cost reductions in the rotary kiln/LMD calcinationprocess appears difficult. There is, however, wasted energy within therotary kiln/LMD calcination process that could be recovered with theproper technical approach. Notably, at higher lime mud solidsconcentration the calciner's exit gas temperature increases. If acounter-current heat transfer process (i.e. a rotary kiln) werethermally balanced the exit gas temperature would remain constant asfuel input was reduced to compensate for the decreased water input.

Further energy efficiency improvement, however, is not likely with therotary kiln/LMD calcination process since a very large non-variable fuelamount is required to compensate for constant radiation losses, providethe constant endothermic heat-of-reaction enthalpy, and also heatreaction products (CaO and CO₂) to the calcination temperature. Thisnon-variable fuel input has associated gaseous fuel combustion productsfrom which enthalpy is recovered via counter-current contact with drylime mud solids in the kiln pre-heat section using densely packedhanging chains as heat transfer surface. This preheats the dry lime mudbefore it enters the following kiln calcination stage.

The reduced temperature gaseous combustion products (and released CO₂)leave the kiln pre-heat section and enter the kiln drying section wherethese gases' enthalpy content evaporates incoming lime mud watercontent. Older kilns have chains within the kiln drying section toimprove gas-to-water heat transfer. Newer kilns with an LMD do not havedrying section chains and are easier to control and operate. Aspreviously stated, as lime mud solids content increases the need fordrying enthalpy decreases. The following kiln pre-heat section, however,has insufficient chain heat transfer ability to absorb availableenthalpy from the combustion products and CO₂ associated with theaforementioned non-variable fuel component and transfer it into thedried solids entering from the drying zone. The unabsorbed enthalpyassociated with the combustion products and CO₂ results in a higher LMDoutlet gas temperature when high solids lime mud is the feedstock. Overthe last forty years, improvements in lime mud filtration and washinghave increased filter cake solids content from 70% to over 85%,resulting in significant fuel savings and improved white liquorrecovery. Unfortunately, the current rotary kiln/LMD technology islimited in the ability to economically respond to this fuel savingopportunity and will become less fuel-efficient as filter cake solidscontent further increases.

The less utilized FluoSolids fluidized bed calcination process neverfeatured a solids pre-heat section, and wastefully dissipated thisexcess available enthalpy via a water spray cooler to control lime mudflash dryer operating temperature. Designs have also been proposed toaddress this dilemma by inserting a waste heat boiler in place of thespray cooler step, but were never commercialized, most likely due to thehigh surface fouling characteristics of calciner exit gas caused by thepresence of low eutectic melting point mixtures of Na₂CO₃ and sodiumNa₂SO₄. The Na₂SO₄ is created by the oxidation of Na₂S₂O₃ and Na₂S inthe air fluidized calciner.

It would, therefore, be beneficial to provide a process whereby gaseousfuel combustion products could be separated from gaseous calcinationreaction products (CO₂) such that the excess enthalpy contained in thecombustion products could be viably extracted as superheated highpressure steam without the presence of heat transfer fouling mixturessuch as Na₂CO₃/Na₂SO₄ This is not possible within the body of a rotarykiln however the disclosed invention, with separated combustion andcalcination stages, addresses this need.

Concurrent with these enthalpy utilization optimization needs, all millsmust control the amount and toxicity of gaseous, liquid, and solidwastes expelled. Many of these emissions have been reduced or eliminatedthanks to better manufacturing practices but WWTP sludge (cellulosic,organic, and inorganic matter from waste water treatment) remains acostly disposal problem since it must ultimately be placed in alandfill. As previously discussed, WWTP sludge cannot be used inexisting rotary kiln representing a lost opportunity to conserve fossilfuels.

Safe disposal of non-condensable waste mill gas (NCGs), which aretypically combusted in the recovery or power boiler, or more likely, therotary kiln lime mud calciner is another Kraft paper pulp milloperability issue. While NCG combustion in rotary kilns has been widelypracticed, operability problems (kiln deposit “ringing”, SO₂“blow-through”, etc.) persist at many mills. Accordingly, stand aloneNCG incinerators with waste heat boilers that raise steam and scrubsulfurous emissions are increasingly used. These incinerator/boilers,however, are not always available when NCGs are produced, so a back-updisposal means is desirable.

Numerous advances have been previously made related to various aspectsof lime mud and limestone calcination and related materials. U.S. Pat.No. 2, 212,446 teaches limestone calcination in a 100% steam atmosphere(a claim of the disclosed invention) using an indirect heated rotarycalciner. U.S. Pat. No. 2,700,592 teaches using moving media heattransfer (MMHT) between an endothermic fluidized bed process and anexothermic fluidized bed sulfide ore roasting process. U.S. Pat. No.2,738,182 teaches fluidized bed calcination of Kraft pulp mill lime mudincluding recycling finely ground calcined lime mud product into acalciner bed to control agglomeration. U.S. Pat. No. 3,961,903 teaches aspray dryer to dry lime mud using multiple hearth calciner off-gases asthe drying medium prior to feeding the dried mud to the calciner. U.S.Pat. No. 3,991,172 teaches direct combustion products calcination offine limestone by passing the limestone through a fluidized bed of a“granular heat carrier medium”. U.S. Pat. No. 4,321,239 teaches usingmultiple spray dryers to dry lime mud using multiple hearth calcineroff-gases as the drying medium prior to feeding the dried lime mud to acalciner. U.S. Pat. No. 4,389,381 teaches using MMHT by passing finelimestone through an inert heat carrier contained in an endothermicfluidized bed and using a coal fueled exothermic fluidized bed tore-heat the heat carrier. Ash is separated from the re-heated heatcarrier prior to calcination. Calcination is accomplished in an airatmosphere of unspecified composition. U.S. Pat. No. 4,606,722 teaches asolid fuel gasified external to a rotary kiln lime mud calciner with thesyngas used as calciner fuel. A vitrified gasifier ash is mixed withcalcine and removed in the slaker. U.S. Pat. No. 4,631,025 teachesdirect injection of a solid fuel (petroleum coke) into a fluidized bedlime mud calciner. U.S. Pat. No. 4,707,350 teaches calcination of finelimestone in an electrically heated fluid bed calciner fluidized in a100% CO₂ atmosphere with recovered CO₂ as the fluidizing gas. U.S. Pat.No. 4,760,650 teaches indirect steam heated drying of lime mud in asteam atmosphere prior to feeding the dried lime mud into a fluid bedcalciner. The steam is generated from calciner off-gas. U.S. Pat. No.5,110,289 uses a separate flash dryer to dry Kraft paper pulp mill limemud using rotary calciner off-gases as the drying medium. U.S. Pat. No.5,230,880 teaches calcination of fine limestone in an electricallyheated fluid bed calciner fluidized in an air atmosphere. The finelimestone is passed through a bed of coarser calcined limestoneparticles that act as a heat transfer media between the fine limestoneand the electric heaters. U.S. Pat. No. 5,354,375 describes a lime mudcalcination process using a shaft kiln to process pelletized lime mud ina counter-current fashion using direct firing of oil or natural gasfuel. U.S. Pat. No. 5,378,319 describes a lime mud calcination processusing an electrically heated microwave belt oven to process lime mud ina counter-current fashion using a counter-current air sweep. U.S. Pat.No. 5,644,996 teaches a technique to cool freeboard gases in a fluidizedbed lime mud calciner to below 500° C. (932° F.) to minimize freeboardscaling when the calciner fluid bed is between 875° C. (1607° F.) and1000° C. (1832° F.). The injected coolant is the entire amount of wetlime mud. U.S. Pat. No. 5,653,948 teaches an indirectly heated fluid bedcalciner using electricity or oil/gas firing to calcine very finelimestone particles. The limestone is injected beneath a coarserlimestone bed that acts as the heat transfer medium. U.S. Pat. No.5,711,802, teaches a technique to reduce the LMD inlet gas temperaturefrom a rotary kiln lime mud calciner to between 400° C. (752° F.) and600° C. (1112° F.); that eliminates dryer scaling and reduces kiln dustcarry-over. United States Patent Application Publication No.2006/0039853 teaches a process to separate CO₂ from utility boiler stackgases with an “activated” CaO sorbent and then separately re-generatingthe sorbent and recovering the CO₂ in a steam blanketed vacuum calciner.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method forrecovering carbon dioxide by

-   -   (a) providing wet calcium carbonate lime mud sufficiently near a        bubbling fluid bed calciner and a spray dryer or flash dryer        such that the calciner and flash dryer or spray dryer operate in        counter/current gas/solids flow wherein exiting calciner gases        substantially dry the wet lime mud and the resulting dry lime        mud is fed to the calciner;    -   (b) feeding substantially dry lime mud to the fluid bed calciner        wherein the fluid bed calciner is thermally linked by moving        media heat transfer (MMHT) to a circulating fluid bed combustor        by a heat transfer media wherein said media moves between said        calciner and said combustor wherein MMHT provides heat input for        calcination;    -   (c) recycling the heat transfer media from the calciner to the        combustor; and    -   (d) recovering excess energy from the process of c) generating        superheated high pressure steam;    -   (e) recovering carbon dioxide and calcined lime mud from the        fluid bed calciner; and, optionally    -   (f) exporting the superheated high pressure steam.

The method may further feature removing substantially all ash from theheat transfer media, for instance, after (b). In some instances, thecombustor receives one or more fuels such as, for instance, Kraft pulpand paper mill sludge, biomass, precipitated lignins and NCGs. In someinstances, the (c) recycling the heat transfer media from the calcinerto the combustor further features producing combustion products. In someinstances, the method provides for recovering carbon dioxide of 90%,95%, 97%, or 99% or more purity and calcined lime mud.

The calcium carbonate lime mud may be obtained from a Kraft pulp andpaper mill. In some instances, steam is provided to the bubbling fluidbed calciner as fluidization and diluent gas, and in some instances hotCO₂, and calcined lime mud are produced in the calciner and mixed withentering fluidization steam. In other instances, the hot CO₂, watervapor, and calcined lime mud exiting said calciner may be provided to aseparator to separate coarse calcined lime mud and the hot CO₂/watervapor mixture and residual fine particle calcined lime mud may beprovided to a spray or flash dryer. Likewise, in some instances themethod further features quenching the hot CO₂, and water vapor mixtureand residual fine particle calcined lime mud with cooled calcined limemud product to a lower temperature in a second cyclone separator beforedirecting the hot CO₂, and water vapor mixture and residual fineparticle calcined lime mud to a flash dryer. Similarly, in someinstances the method further features separating CO₂ in the exitingspray or flash dryer dust collector gases from the water vapor bycondensing the water vapor by contact with water in a cooling tower. Thehot process water created by water vapor condensation may be returnedto, for instance, a manufacturing operation or facility such as a Kraftpulp and paper mill manufacturing operation. Also, in some instances,the bubbling fluidized bed calciner contains reheated media particlesreturning from the combustor. The method may also feature providing amakeup media for said moving media to the circulating fluid bedcombustor.

In some instances, the method may further feature providing sorbentlimestone to the circulating fluid bed combustor, to neutralize fuelderived SO₂ emissions. Similarly, the method may feature providing afossil fuel, such as, for instance, coal, petroleum coke, waste coal andshredded tires to the combustor. The superheated high pressure steam maybe generated by heat exchange with hot combustion products. Also, themethod may feature preheating air entering the combustor and calcinerfluidizing steam by heat exchange with hot combustion products. Stillfurther, the method may feature recovering heat as hot process water orboiler feed water from the calcined lime mud. Even further, the methodmay feature mixing the-wet lime mud feed with at least one of water,H₂O₂, O₂, Na₂CO₃, or Na₂SO₄. Also, the method may feature injecting drylime mud into the fluidized media bed of the fluid bed calciner.

In some instances, the method includes removing ash or substantially allthe ash from the heat transfer media, and this may be performed in acombustor freeboard section having an expanded diameter by decreasingvelocity of the heat transfer media. The ash may be removed from theheat transfer media at a temperature of between 1550° F. and 1700° F.,and the ash may be removed by introducing steam into a cone cap andslope stripper. The moving media may move between the calciner and thecombustor at, for instance, 1530° F. to 1700° F., the combustor mayoperate at, for instance, 1550° F. to 1700° F., and the calciner mayoperate at 1400° F. to 1570° F., and the superheated high pressure steamgenerated may have a temperature of 750° F. to 1000° F. at a pressure of615 psia to 1515 psia.

In a second aspect, the present invention provides a method forcalcining calcium carbonate lime mud and converting it to carbon dioxideand calcined lime mud comprising:

-   -   (a) providing wet lime mud sufficiently near a bubbling fluid        bed calciner and a spray dryer or flash dryer such that the        calciner and flash dryer or spray dryer operate in        counter/current gas/solids flow wherein exiting calciner gases        substantially dry the wet lime mud and the resulting dry lime        mud is fed to the calciner;    -   (b) feeding substantially dry lime mud to the fluid bed calciner        wherein the fluid bed calciner is thermally linked by moving        media heat transfer (MMHT) to a circulating fluid bed combustor        by a heat transfer media wherein said media moves between said        calciner and said combustor wherein MMHT provides heat input for        calcination;    -   (c) removing substantially all ash from the heat transfer media;    -   (d) recycling the heat transfer media from said calciner to said        combustor wherein said combustor receives one or more fuels and        producing combustion products;    -   (e) recovering excess energy from the process of d) generating        superheated high pressure steam;    -   (f) recovering carbon dioxide of 90% to +99% purity and calcined        lime mud from the fluid bed calciner; and    -   (g) exporting the superheated high pressure steam to the Kraft        pulp and paper mill.

In some instances, steam is provided to the bubbling fluid bed calcineras fluidization and diluent gas, and in some instances hot CO₂, andcalcined lime mud are produced in the calciner and mixed with enteringfluidization steam. In other instances, the hot CO₂, water vapor, andcalcined lime mud exiting said calciner may be provided to a separatorto separate coarse calcined lime mud and the hot CO₂/water vapor mixtureand residual fine particle calcined lime mud may be provided to a sprayor flash dryer. Likewise, in some instances the method further featuresquenching the hot CO₂, and water vapor mixture and residual fineparticle calcined lime mud with cooled calcine product to a lowertemperature in a second cyclone separator before directing the hot CO₂,and water vapor mixture and residual fine particle calcined lime mud toa flash dryer. Similarly, in some instances the method further featuresseparating CO₂ in the exiting spray or flash dryer dust collector gasesfrom the water vapor by condensing the water vapor by contact with waterin a cooling tower. The hot process water created by water vaporcondensation may be returned to a manufacturing process or operation,for instance, a Kraft pulp and paper mill manufacturing operation. Also,in some instances, the bubbling fluidized bed calciner contains reheatedmedia particles returning from the combustor. The method may alsofeature providing a makeup media for said moving media to saidcirculating fluid bed combustor.

In some instances, the method may further feature providing sorbentlimestone to the circulating fluid bed combustor, to neutralize fuelderived SO₂ emissions. Similarly, the method may feature providing afossil fuel, such as, for instance, coal, petroleum coke, waste coal andshredded tires to the combustor. The superheated high pressure steam maybe generated by heat exchange with hot combustion products. Also, themethod may feature preheating air entering the combustor and calcinerfluidizing steam by heat exchange with hot combustion products. Stillfurther, the method may feature recovering heat as hot process water orboiler feed water from the calcine. Even further, the method may featuremixing the-wet lime mud feed with at least one of water, H₂O₂, O₂,Na₂CO₃, or Na₂SO₄. Also, the method may feature injecting dry lime mudinto the fluidized media bed of the fluid bed calciner.

In some instances, removing ash or substantially all the ash, from theheat transfer media may be performed in a combustor freeboard sectionhaving an expanded diameter by decreasing velocity of the heat transfermedia. The ash may be removed from the heat transfer media at atemperature of between 1550° F. and 1700° F., by introducing steam intoa cone cap and slope stripper. The moving media may move between thecalciner and the combustor at, for instance, 1530° F. to 1700° F., thecombustor may operate at, for instance, 1550° F. to 1700° F., thecalciner may operate at 1400° F. to 1570° F., and the superheated highpressure steam generated may have a temperature of 750° F. to 1000° F.at a pressure of 615 psia to 1515 psia.

In a third aspect, the present invention features a process forproducing carbon dioxide comprising:

-   -   (a) feeding lime mud obtained from a recausticizing operation to        a bubbling fluid bed calciner thermally linked by moving media        heat transfer (MMHT) using a solid particulate media to a second        circulating fluid bed combustor wherein the MMHT provides heat        input for calcining the lime mud;    -   (b) recycling the solid particulate media being from said        calciner to said combustor; and    -   (c) recovering carbon dioxide and calcined lime mud from the        bubbling fluid bed calciner.

In some instances the carbon dioxide produced is at least 90%, at least95%, at least 97%, at least 99% or more pure. In some embodiments, theprocess features further after step a) using calcination gases to drywet lime mud from the recausticizing process in a spray dryer or flashdryer. In still further embodiments, the process features after step a)recovering excess enthalpy from the combustor process as super heatedhigh pressure steam. The dry lime mud may be obtained from a spray orflash dryer, and the circulating fluid bed combustor may be providedWWTP sludge, biomass, precipitated lignins or NCGs as fuel. Steam may beprovided to the fluid bed calciner as fluidization and diluent gas. Thesteam may also serve to increase the calcination reaction. Hot CO₂,steam, and calcined lime mud is normally produced from the fluid bedcalciner and in most instances provided to a cyclone separator. Acyclone separator may separate coarse calcined lime mud and feed the hotCO₂, steam and residual fine particle calcined lime mud to a spray dryeror a flash dryer for said lime mud. In some instances, a spray dryer isused to produce high purity CO₂ and a flash dryer is used to produce alower quality CO₂ In other embodiments, the method features quenchingthe hot CO₂, and steam mixture and residual fine particle calcined limemud with cooled calcined lime mud to a lower temperature in a secondcyclone separator before directing the CO₂, and steam mixture andresidual fine particle calcined lime mud to a flash dryer.

In preferred embodiments, the process features additionally providingwet lime mud sufficiently near the fluid bed calciner and a spray dryeror flash dryer such that exiting gases from the calciner substantiallydry the wet lime mud and the resulting relatively dry lime mud is fed tothe calciner. The CO₂ in the exiting calciner gases may be separatedfrom the combined fluidization steam and spray or flash dryer exitingwater vapor by condensing this total water vapor amount with liquidwater using direct contact in a cooling tower. In some embodiments, thehot liquid water created by water vapor condensation, and the quenchingof exiting calciner CO₂ may be returned to the re-causticizing circuit.

In yet other embodiments, the method makes use of a calciner and a spraydryer or flash dryer operating in countercurrent gas/solids flow whereinwet lime mud is dried by exiting calciner gases and the resulting drylime mud is fed to the calciner. In some embodiments, the fluid bedcalciner is a bubbling fluidized bed calciner wherein the bed maycomprise reheated solid particulate media returning from the circulatingfluid bed combustor. In some embodiments, the method features feeding amakeup media for said moving media to said circulating fluid bedcombustor. The makeup media may be alumina, silica, mullite or othersolid, inert materials noted for strong thermal cycling and mechanicalstrength characteristics. In additional embodiments, the method featuresproviding a sorbent limestone to said circulating fluid bed combustor.Such sorbent limestone may be useful to neutralize fuel derived SO₂. Inyet other embodiments, the combustor may have a second fuel sourceincluding higher cost fossil fuels. Such fossil fuels may serve asbackup fuels in the event of unavailability of adequate said primaryfuels.

In some embodiments the method features generating superheated highpressure steam by heat exchange with hot combustion products. Thesefeatures may allow export of said super heated high-pressure steam to asteam system of a mill. In addition, in some instances, the methods andsystems may feature preheating combustion air for the circulating fluidbed combustor by heat exchange with hot combustion products. Inaddition, in some instances, the methods and systems may featuresuperheating saturated low pressure steam imported from the mill toprovide the calciner process with internal steam requirements. Further,the methods and systems may include recovering enthalpy from thecalcined lime mud using an indirect water cooled fluid bed device togenerate hot boiler feed-water or hot process water.

Additionally, the method may include mixing the wet lime mud feed to thespray dryer or flash dryer with hydrogen peroxide (H₂O₂) or O₂, toconvert Na₂S contained in said lime mud into Na₂SO₄. Aqueous solutionsof Na₂CO₃, or Na₂SO₄ may also be mixed to alter the Na₂CO₃/Na₂SO₄ ratiowithin the lime mud to a higher melting point. This may be particularyeffective to mitigate calciner scaling and fouling and unwanted gaseousemissions. Still further, the method may include injecting the dry limemud feed to the calciner at the base of the fluidized media bed of thecalciner. This may help to maximize dry lime mud particle residence timethereby ensuring thorough calcination.

In a fourth aspect, the present invention provides a system forrecovering carbon dioxide, for calcining calcium carbonate lime mud andconverting it to carbon dioxide and calcined lime mud, or for calciningcalcium carbonate lime mud and converting said calcium carbonate limemud to CO₂ and calcined lime mud comprising a bubbling fluid bedcalciner thermally linked by MMHT to a second circulating fluid bedcombustor and wherein the MMHT provides heat input for calcining thelime mud. The system comprises a calciner and a combustor linked by aMMHT system or apparatus. The MMHT system or apparatus thermally linksseparate fluid bed combustion (exothermic) and calcination (endothermic)stages. In some embodiments, the system further comprises a spray dryeror a flash dryer.

In a fifth aspect, the present invention provides an integrated systemcomprising five interconnected, pyroprocessing and heat exchange unitoperations, namely lime mud drying, bubbling fluid bed dry lime mudcalcination, bubbling fluid bed calcined lime mud cooling, direct mediaheating within a circulating fluid bed combustor, and combustionproducts heat recovery and steam generation. The calciner and dryeroperate in countercurrent gas/solids flow with wet lime mud being driedby exiting calciner gases and the resultant dry mud then being fed tothe calciner. The present system provides MMHT to thermally linkseparate fluid bed combustion (exothermic) and calcination (endothermic)stages. A high temperature media, when separated from contaminates, istransported to a lower operating temperature bubbling fluid bed calcinerwhere it surrenders stored enthalpy to satisfy the calciner andpreceding lime mud dryer's endothermic heat needs. The cooled mediaexiting the calciner is then returned to a combustor for reheating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 provides a schematic block diagram of a preferred system of theinvention.

FIG. 2 provides a schematic block diagram of a preferred system of theinvention.

FIG. 3 depicts a further schematic block diagram providing more detailfor the system of the present invention.

FIG. 4 depicts a further schematic block diagram providing additionaldetail for the system of the present invention.

FIG. 5 depicts a further schematic block diagram providing additionaldetail for the system of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Unless otherwise specified, as used herein, the following terms mean thefollowing:

By “lime mud” is meant a water-wet lime mud produced as a fineprecipitated CaCO₃ particle reaction product in a recausticizingmanufacturing step where NaOH is made from the recausticizing reactionof CaO and Na₂CO_(3.)

By “waste water treatment (WWTP) sludge” is meant a primarily negativevalue water-wet sludge fuel as produced in a Kraft paper pulp mill'swaste-water treatment plant (WWTP). This sludge contains organic, andinorganic, materials that may be rejected from various pulping and papermaking steps. The energy content may be in the form of organiccompounds, primarily cellulosic fiber and rejected lignins.

By “biomass” is meant a positive value fuel and may consist of bark,field trimmings, etc. derived from a mill's raw fiber feedstock (trees).The term also includes precipitated lignins.

By “non-condensable gases (NCGs)” is meant a noxious and explosive mixof mercaptans, hydrogen sulfide, methyl sulfides, turpentine, andmethanol collected from various Kraft paper pulp mill processes, e.g.blow heat recovery systems, turpentine recovery, etc. NCGs may becombusted in a recovery boiler, or an incinerator/boiler, both toeliminate the nuisance and to recover enthalpy and valuable sulfur.

By “calcination” is meant a high temperature endothermic (heat is addedto drive a chemical reaction) industrial thermal process to thermallydissociate inorganic carbonates (i.e., calcium and magnesium carbonates,a.k.a., limestones and dolomitic limestones) and hydroxides (i.e.,aluminum and magnesium hydroxides) into the reactive, solid calcium,magnesium, or aluminum oxides and liberated gaseous reaction products,water vapor and/or CO₂. For example, lime mud may be first dried andthen dissociated into CO₂ and CaO (a.k.a., calcined lime mud) with thelatter being recycled to a preceeding recausticizing manufacturingoperation. The gaseous, liberated CO₂, is that which originated with theNa₂CO₃.

By “calciner” is meant a chamber or apparatus for conducting acalcination reaction. A “calciner” may be fueled with oil, natural gas,or in some cases, coal or biomass, and, dependent on the feedstock'sphysical state. A “calciner” may be a shaft kiln, rotary kiln, flashcalciner or fluid bed calciner as manufactured by Metso Mining or amultiple hearth furnace as manufactured by Hankin-Nichols.

By “bubbling fluid bed calciner” is meant a calciner that uses a solidparticulate heat transfer media (e.g., silica, alumina, mullite, etc.)to calcine dried lime mud, all of which being suspended (i.e.,fluidized) in an upward flowing stream of steam. The upward gas velocitymay allow mixing all the solids with steam and evolved CO₂ “bubbles,”but is usually not high enough to transport the media out of thebubbling bed but still allowing the calcined lime mud particle to betransported out of the bubbling media bed.

By “combustion” is meant an oxidative combustion (i.e., exothermic)reaction to release enthalpy contained in fuels (WWTP sludges, biomass,coal, coke, etc.) by mixing the fuels with excess air (the oxygensource). “Combustion” is a widely used high temperature industrialthermal process used to create recoverable enthalpy from the fuelcombustion products (CO₂, nitrogen, oxygen, and water vapor). Theextracted enthalpy may then be used for a final endothermic purpose,i.e., calcination, heating fluids, drying, generating steam, etc.

By “combustor” is meant a chamber or apparatus that conducts acombustion reaction under controlled conditions and permits thecontrolled extraction of the liberated enthalpy for useful processpurposes. There are numerous industrial combustors available. Theoptimal choice is determined by fuel type and the desired end use of theenthalpy, i.e., steam generation, hot water production, gases (air,etc.) heating, process heat transfer fluid heating, or solid particulatemoving heat transfer media heating.

By “moving media heat transfer (MMHT)” is meant a means or process fortransferring the enthalpy generated by solid fuel combustion process inone vessel to an endothermic calcination reaction in another vessel. Forexample, typical calcination processes utilize in situ (in the samevessel as calcination) combustion. Many of these calcination processesuse ash-free, higher cost liquid and gaseous fuels (oil, natural gas)since ashes contained in much lower cost solids fuels would contaminatethe calcined lime mud. MMHT allows low cost fuel use while avoidingcalcined lime mud contamination with ash. By using MMHT, a calciner'sendothermic reaction enthalpy may be transported to the calciner by afluidizable solid particulate media that freely circulates between acombustor and a calciner. Many low cost solid fuels are effectivelycombusted in a circulating fluid bed combustor, and many calcinationreactions effectively occur in bubbling fluidized bed reactors. Thereby,MMHT provides for extracting released enthalpy in a combustor and thentransferring that same enthalpy into a calciner. In exemplary MMHTsystems, the solid particulate heat transfer media has a larger sizeparticle than the entering dried lime mud such that the dried lime mudpasses through the bubbling fluidized media bed and is fully calcined,without cross-contamination. The calciner's operating temperature islower than the combustor's such that the contained media's enthalpy canbe rapidly transferred into the calciner and then re-heated in thehigher temperature combustor. In other exemplary systems, MMHTfacilitates steam fluidization of the calciner which permits loweringthe calcination temperature thereby creating an effective temperaturedifferential between the combustor and calciner which further reducesthe particulate heat transfer media's mass circulation rate. Lastly,MMHT facilitates calcination atmosphere control by allowing flexibleCO₂/steam ratio partial pressure adjustment and temperature adjustmentby separating the combustor's gaseous combustion products from thecalciner's CO₂/steam gaseous atmosphere.

By “circulating fluid bed combustor” is meant a combustor designfeaturing a circulating fluid bed combustor comprised of two fluidizedmedia sections. A lower bubbling fluid bed section contains largeparticle heat transfer media fluidized by incoming fluidization air.Fine particle media returning from the calciner and fuel may be injectedinto this large media bed. The air velocity is normally insufficient totransport large media particles out of this bed, but sufficient tovertically transport fine particle media and fuel ash media out of itinto a second section, known as the transport column. Fine media withinthis latter section absorbs much of the released fuel enthalpy. Some ofthe fine media contacting the transport section's wall may circulateback to the lower bed. This design approach is very similar to that usedfor circulating fluid bed boilers as manufactured by Foster Wheeler orAlstom.

By “spray dryer” is meant a convective dryer that may be fed with, forexample, pumpable slurries, pastes, or solutions that may be atomizedinto a fine, spherical droplet “cloud” by a mechanical atomizer, such asa high speed rotating disc or pressure nozzle. The cloud may becontacted with a hot gas stream capable of evaporating moisture from thepumpable feed. Exemplary spray dryers include those manufactured by GEANiro.

By “flash dryer” is meant a convective dryer that may be fed with filtercakes, sludges, or fibrous materials. The feed may be introducedproximate to a drying gas introduction point such that dried solids andspent drying gases may move in a co-current fashion in a verticaltransport column. Such a co-current flow profile may limit the inlet gastemperature dependent on the heat sensitivity of the feed material.Flash dryers include those manufactured by Alstom.

The present methods and systems produce commercial quality CO₂ of 90% to+99% purity. The commercial quality CO₂ may be produced using, forinstance, Kraft paper pulp mill lime mud as the sole feed material. Highreactivity “soft-burned” calcined lime mud product required in the pulpmill's recausticizing circuit is also produced in an energy efficientmanner by utilizing readily available low quality and low cost fuels.

The present methods and systems utilize two particularly keytechnologies, namely (1) MMHT and (2) steam calcination. The methods ofthe present invention rely on MMHT to thermally link separate fluid bedcombustion (exothermic) and calcination (endothermic) stages. Thisallows using low cost mill waste and biomass fuels without contaminatingcalcined lime mud with fuel ash. The methods of the present inventionresult in high quality CO₂ recovery by not commingling ash or combustionproducts with fluidization steam and CO₂ evolved during the calcinationreaction. The methods also provide high quality calcined lime mud sincethere is no commingling of ash with the calcined lime mud created by thecalcination reaction.

By using MMHT, unlike the rotary kiln/LMD process, required enthalpy forlime mud calcination and drying is not generated in situ within thecalciner. Instead, a separate circulating fluid bed combustor bumsnegative value mill (WWTP) sludge and noxious NCGs with readilyavailable, higher quality biomass (bark, tree trimmings, sawdust, etc.)fuel to heat circulating inert, solid particulate media. This hightemperature media, when separated from ash contaminates, is thentransported to the lower operating temperature bubbling fluid bedcalciner where it surrenders its stored enthalpy to satisfy the calcinerand preceding lime mud dryer's endothermic enthalpy needs. The cooledsolid particulate media exiting the calciner is then returned to thecombustor for reheating.

WWTP sludge is not used in a rotary kiln/LMD lime mud calciner given itshigh ash and moisture content. While ash contamination alone is a majorimpediment, WWTP sludge's high moisture content precludes creation ofthe high adiabatic flame temperatures required in a rotary kiln foreffective heat transfer and flame stability.

A strong enthalpy balance relationship exists, however, which, whencombined with fluid bed combustion and MMHT, justifies using WWTP sludgein a calcination process provided there is a significant operatingtemperature differential between the combuster and calciner media beds.A typical bleached Kraft paper pulp mill requires 320 mtpd of calcinedlime mud/1000 adtpd paper pulp. Additionally, a typical mill producesapproximately 100 mtpd dry basis WWTP sludge per 1000 adtpd paper pulp.Therefore, the dry basis WWTP sludge to calcined lime mud mass ratio, onan equivalent basis, is 0.313 dry mtpd WWTP sludge/mtpd calcined limemud.

A typical modern rotary kiln/LMD) has a high heat value (HHV) fuel oilconsumption of 1.4 million Kcal/metric ton calcined lime mud. Wet (58%water) WWTP sludge has an HHV of 2159 Kcal/kg. Therefore, given the0.313 dry mtpd WWTP sludge/mtpd calcined lime mud mass ratio, the totalenthalpy available in wet WWTP sludge per ton of calcined lime mud is1.61 million Kcal, or a significant portion of the required net calcinerenthalpy load even when considering that enthalpy value lost toevaporating WWTP sludge associated water.

The most common application of MMHT is in a petroleum refinery's fluidcatalytic cracking operation where a liquid semi-refined feedstock isthermally “cracked” into gasoline feedstock in a fluid bed reactorutilizing a hot re-circulating solid catalyst to transfer enthalpy todrive the endothermic cracking reaction.

The methods described herein also use steam to control calciner CO₂partial pressure while also allowing the calcination reaction to proceedmore rapidly than that found with air based calcination while alsoproviding an easy means to subsequently separate commercial quality CO₂from the steam using accepted steam condensation techniques. Loweringthe calcination temperature facilitates MMHT use by creating asatisfactory temperature differential between the hot media and thecalcination temperature thereby permitting rapid heat transfer to occurat reduced media circulation rates between combustor and calciner.Lowering the calcination temperature also inhibits calcined lime mudagglomeration and calciner surface scaling caused by low melting pointmixtures of Na₂CO₃ and Na₂SO₄. Conversely, MMHT facilitates the flexibleadjustment of steam/CO₂ mixtures and temperatures in the calciner by notallowing gaseous combustion products to mix with the calcinationatmosphere.

The favorable effect of steam on calcium carbonate's calcinationreaction rate was first noted by Berger (“Effect of Steam on theDecomposition of Limestone”, Industrial and Engineering Chemistry, vol.19, no. 5, May 1927). Senum (Steam Catalysis of Limestone Calcination,Brookhaven National Laboratory, Contract EY-76-C-02-0016 for US ERDA,1976) provided a good overview of early, significant research with themost relevant references to the present invention being Bischoff(“Kinetics of Thermal Dissociation of Dolomite and Limestone in VariousGas Flows”, Zeitschrift für Anorganische Chemie., vol. 262, 1950) andMaclntire/Stansel (“Steam Catalysis in Calcination of Dolomite andLimestone Fines”, Industrial and Engineering Chemistry, vol. 45, no. 7,July 1953). Subsequently, Burnham/Stubblefield/Campbell (“Effects of GasEnvironment on Mineral Reactions in Colorado Oil Shale”, Fuel, vol. 59,December 1980), Weisweiler/Hoffman (“Effect of Water Vapour on theCalcination of Limestone in a Fluidized Bed Reactor”, Zement-Kalk-Gips(36, Jahrgang), nr. 10, 1983), Khraisha/Dugwell (“Effect of Water Vapouron the Calcination of Limestone and Raw Meal in a Suspension Reactor”,Transactions of the Institute of Chemical Engineers, vol. 69, part A,January 1991), and Wang/Thompson (“The Effects of Steam and CarbonDioxide on Calcite Decomposition using Dynamic X-Ray Diffraction”,Chemical Engineering Science, vol. 50, no. 9, 1995) provided greaterinsight.

More recently, and due to the high interest in capturing CO₂ from coalfired power plants to remediate global warming, there has been anincrease in the knowledge base regarding limestone calcination in steamatmospheres with the most relevant references to the present inventionbeing Wang et. al. (“Limestone Calcination with CO₂ Capture (II):Decomposition in CO₂/Steam and CO₂/N₂ Atmospheres”, Energy and Fuels,vol. 22, no. 4, 2008), Wang et. al. (“Experimental Study on CO₂ captureConditions of a Fluidized Bed Limestone Decomposition Reactor”, FuelProcessing Technology, vol. 91, pages 958-963, 2010) and Feng et. al.(“Modeling of CaCO₃ Decomposition under CO₂/H₂O Atmospheres in CalciumLooping Processes”, Fuel Processing Technology, vol. 125, pages 125-138,2014).

A consensus of these research results is that using a 100% steamatmosphere or steam/air atmospheres of certain ratios result in three(3) key impacts during limestone (CaCO₃) calcination; (1), a lowering ofthe equilibrium dissociation temperature at the same CO₂ partialpressure relative to that in 100% air or N₂ and (2), a limited catalyticeffect, primarily at lower calcination temperatures with an increase inthe calcination reaction rate and (3), any positive reaction rate effectdiminishes after a maximum steam concentration is attained.

In the present invention, hot media is introduced into the calcineralong with dry lime mud. Also sufficient fluidizing steam to catalyzethe calcination reaction, and control calciner temperature and CO₂partial pressure is injected. All these components are thoroughly mixedin the back-mixed, bubbling fluid bed calciner. Reaction productsconsisting of calcined lime mud particles and the gaseous steam/CO₂mixture exiting the fluid bed calciner are cyclone separated before thelargely cleaned, hot, steam/CO₂ mixture reports to the lime mud dryer.Reduced temperature media is gravity discharged from the fluid bedcalciner and returned to the combustor to renew the heating cycle.

To inhibit calcine agglomeration and calciner surface scaling due to thepresence of low melting point mixtures of Na₂CO₃ and Na₂SO₄, a portionof the hot calcined lime mud product is recycled to the calciner tocreate more nucleation sites, thereby reducing the potential forcalcined lime mud particle agglomeration and calciner surface scaling.To also minimize introduction of low melting point Na₂CO₃/Na₂SO₄mixtures, prior to the calciner, H₂O₂ is added to wet lime mud (prior tofeeding same to the flash or spray dryer) to convert residual Na₂S toNa₂SO₄. This creates a higher melting point Na₂CO₃/Na₂SO₄ mixture. Smallamounts of Na₂SO₄ or Na₂CO₃ solutions may be added at the same point tohave the same result.

The calcination and lime mud drying steps in the present invention areprocess decoupled from the combustion step. This feature of the presentinvention provides the opportunity for substantial process controlwithin the calciner since the calcination atmosphere can be carefullymodeled without the influence of fuel combustion products. The exittemperature and humidity of the lime mud dryer may also be optimized formaximum energy efficiency by balancing its enthalpy needs with thecalciner atmosphere's exiting steam/CO₂ mixture temperature and volume.

The calcination temperature can be varied between 760° C. (1400° F.) and854° C. (1570° F.) by altering the media circulation rate between thecalciner and the combustor. Considering this separation of unitoperations, the CO₂ partial pressure exiting the calciner can also bevaried between 25% and 90%, but typically 85%, of the dissociationequilibrium CO₂ partial pressure for calcination within a givensteam/CO₂ atmosphere.

Within certain combinations of calcination reaction CO₂ partial pressureand temperatures, the calcination reaction rate may be significantlydepressed. To ensure complete calcination, the dry lime mud may beinjected into the base of the calciner, beneath the bubbling media bed,yielding a “hindering effect’ of the larger media particle bubbling bedon an upward flowing smaller lime mud particle thereby providingenhanced residence time. The dense media bed acts as a physical barrierto prevent un-calcined lime mud particles from exiting the calciner tooquickly. See, Talukdar/Mathur, (“Residence Time Studies of FineParticles Circulating through a Fluidized Bed of Coarse Solids”.Department of Engineering, University of New Hampshire. Presented atAIChE 1995 Annual Meeting).

The total enthalpy of exiting calcination gases may be sufficient toefficiently dry the incoming lime mud in a spray dryer at an inlet gastemperature not less than 760° C. (1400° F.) and not exceeding 982° C.(1800° F.) while maintaining a spray dryer exit gas temperature at noless than 88° C. (190° F.) and no greater than 104° C. (220° F.)depending upon the entering total lime mud solids content, but with theneed to maintain a dried particle moisture content at no greater than 2%by weight.

Further, the total enthalpy of exiting calcination gases may also besufficient to efficiently dry the incoming lime mud in a flash dryer atan inlet gas temperature not exceeding 593° C. (1100° F.) whilemaintaining a flash dryer exit gas temperature at no less than 91° C.(195° F.) and no greater than 104° C. (220° F.) dependent upon theentering total lime mud solids content, but with the need to maintain adried particle moisture content at no greater than 2% by weight. Thisdrying gas inlet temperature control is necessary to prevent lime mudagglomeration in the flash dryer, and/or mechanical damage to the flashdryer and is accomplished by quenching calciner hot exit gases withre-cycled, cool calcined lime mud from the calcined lime mud cooler andreturning re-heated calcined lime mud to the cooler.

Fuel ash and combustion products are not mixed with solid and gaseousreaction products in the present methods and systems and unlike therotary kiln/LMD processes, high cost liquid/gaseous fossil fuels are notused. Decoupling also permits the relatively clean hot, gaseouscombustion products to transfer enthalpy into combustion air preheatingand steam generation using conventional heat exchanger designs withoutthe unwanted heat transfer fouling influence of the Na₂CO₃/Na₂SO₄ lowmelting point mixtures potentially found in the calcination stage.

MMHT allows cost effective equipment design. The large exhaust gasvolume from the rotary kiln/LMD lime mud calciner largely derives fromfuel combustion products and not the commingled CO₂ reaction product.Decoupling fuel combustion from the fluid bed calciner reduces calcinerexit gas volume thereby significantly increasing the calcined lime mudproduction rate per unit fluid bed area at the same superficialfluidization velocity. Further, the separated circulating fluid bedcombustor is free to operate at a much higher fluidization velocity thanthe separated bubbling fluid bed calciner. Accordingly, the presentmethods and systems allow a compact fluid bed calciner with calcinedlime mud product throughputs equivalent to and perhaps greater than thelargest rotary kiln/LMD systems but without the land usage penalty.

Hydrocarbon cracking and pyrolysis processes typically use MMHT totransfer enthalpy from a solid to a gaseous stream whereas the presentmethods and systems are designed to transfer enthalpy from one fluidizedsolid to another. The media used for MMHT is selected for its thermalstability and resistance to mechanical decrepitation. Inert materialssuch as alumina, silica, and mullite are several examples. The selectedmedia's size distribution and specific gravity is such as to allowvertical transport (with fine fuel ashes) at superficial gas velocitiesbetween 3.1 and 6.1 meters/sec (10 to 20 feet/second) in the combustorwhile also developing a bubbling, dense fluid bed in the calciner atsuperficial gas velocities less than 1.5 meters/second (5 feet/second).The small, dried lime mud particles have a transport velocity well under1.5 m/s, allowing them to transit through the bubbling media bed,absorbing enthalpy from the media, and undergoing calcination beforeexiting with the steam/CO₂ gas mixture.

MMHT combined with the associated separation of the combustion andcalcination processes allows using 100% steam as a calciner atmospherediluent and fluidizing gas instead of non-condensable fuel combustionproducts or just air. Variable steam dilution controls CO₂ partialpressure that impacts calcination reaction rate. Steam also catalysesthe calcination reaction such that it proceeds at a lower temperaturethan if the fluidization gas was only air at the same CO₂ partialpressure. The total volumetric amount of steam also controls CO₂ partialpressure such that the calcination reaction rate can occur at anacceptable level while resulting in a steam/CO₂ mixture that providessufficient enthalpy in the exiting calciner gases to efficiently dryincoming wet lime mud fed to the spray or flash dryer. Importantly,using steam permits subsequent economic separation and recovery ofcommercial quality non-condensable CO₂ from steam using well acceptedcondensation technologies.

The present invention provides an integrated process system comprisingfive (5) separate, but interconnected, pyro-processing and heat exchangeunit operations; (1) lime mud drying, (2) bubbling fluid bed dry limemud calcination, (3) bubbling fluid bed calcined lime mud cooling, (4)direct media heating within a circulating fluid bed combustor, and (5)combustion products heat recovery and steam generation. For energyeconomy reasons, the calciner and dryer operate in countercurrentgas/solids flow with wet lime mud being dried by exiting calciner gasesand the resultant dry mud then being fed to the calciner.

FIGS. 1 and 2 schematically illustrate a system 8 which may be used topractice the present invention. FIGS. 3, 4, and 5 represent aconsiderably more detailed showing of the system 8. FIG. 1 shows thecombustion, heat recovery, calcination and product cooling section 10 ofsystem 8. FIG. 2 shows the dryer and CO₂ preparation section 12 ofsystem 8.

FIG. 2 depicts lime mud 151 from the mill (not shown) provided to dryerfeed preparation means 150 along with hydrogen peroxide 153, and sodiumsulphate or carbonate solution 154 and re-cycled dried lime mud, 195when a flash dryer 185 is used or hot dilution water 149 when a spraydryer 127 is used. Prepared dryer feed then proceeds to the spray dryer127 via 129 and to the flash dryer 185 by 190. The flash dryer 185receives hot gas 187 from temper cyclone 184 and the spray dryer 127receives hot gas 124 directly from calcined lime mud cyclone 121 (FIG.1). Dry lime mud 91 collected from the gas cleaner 136 proceeds tocalciner 66 (FIG. 1). Dryer exit gas 135 from either flash dryer orspray dryer is cleaned by gas cleaner136, and the output gases 139consisting of hot, clean CO₂ and steam are passed to CO₂ cooling means141. The CO₂ and steam gases are direct water 142 quenched and saturatedhot water from the cooling means 141 is passed back through line 149 foruse as hot dilution water when a spray dryer 127 is used, or for returnto the mill 148 when either a spray 127 or flash dryer 185 is used.Cooled, water-saturated CO₂ proceeds 144 to a CO₂ gas processing unitbeyond the battery limits of this invention.

FIG. 1 depicts a combustor 15 fired at 35 with backup coal, petroleumcoke, shredded tires, waste coal, oil and or gas, but uses as itsprimary fuels, WWTP sludge and/or precipitated lignins 14 and/or biomass(bark, sawdust, etc.) 30 and NCG 39 and limestone at 44 to react withexcess SO₂ Make up media 37 for the MMHT is added as needed. The hotmedia 65 passes to calciner 15 and after transferring its enthalpycontent is returned as cool media 57 to combustor 15 for further heatingand recycling. Spent combustion products and ash exit combustor 15 via62 and report to the heat recovery means 80.

Hot combustion products 62 exiting fluid bed combustor 15 enter the heatrecovery means 80 which is provided with saturated steam from the Kraftpaper pulp mill at 81, re-heated boiler feed-water at 104, and ambientcombustion air at 100. Recovered heat is created in the form ofsuperheated high pressure steam 85 exported to the mill, pre-heatedcombustion air 55, pre-heated calcination fluidization steam 83, andinternal service steam 59, all leaving the heat recovery means 80.

Calciner 66 receives super-heated fluidization steam 83 from heatrecovery means 80. The calciner off-gas 119 consisting of hot CO₂, steamand calcined lime mud products, proceeds to hot cyclone separator121.Hot calcined lime mud 122 passes to calcined lime mud cooler 123 andsome residual un-calcined lime mud and calcined lime mud is recycled at93 to calciner 66. Steam, CO₂ and some residual calcined lime mud 124from the hot cyclone 121 pass directly to a spray dryer 127 but may befirst passed to temper cyclone 184 (FIG. 2) when a flash dryer 185 isused.

Fluid bed calcined lime mud cooler 123 is seen to have as indirectcooling inputs boiler feed water 168 from the mill, cool water 171 fromthe mill, fluidization steam 85 and fluidization air 165. Its outputsinclude reheated boiler feed water 104 which is fed to heat recoverymeans 80 and also in part returned 169 to the mill; a hot water returnat 173, cooled calcined lime mud product at 182 to the mill; and cooledcalcined lime mud recycle at 183 to the temper cyclone 184 when a flashdryer 185 is used. Reheated calcined lime mud 186 leaving the tempercyclone 124 and hot calcined lime mud 122 from hot cyclone 121 are bothreturned to the fluid bed calcined lime mud cooler 123. Cleaned, fluidbed calcined lime mud cooler exhaust gases at 176 are sent toatmosphere. Cooled combustion products 112 from heat recovery means 80are sent to a bag house filter 113 at which dry ash 114 is separatedfrom combustion products 112 and added to dry ash 71 from combustor 15and are then subsequently disposed. Cleaned, cool combustion products at115 are sent to atmosphere.

A more detailed diagram is provided in FIGS. 3, 4, and 5 which are notto scale and the process stream numerical designations may notnecessarily follow in the same sequence as the following description.Stream numbers are denoted by “[]” and process equipment items by “( )”.The process depicted in FIGS. 3, 4, and 5 may be divided into eight (8)process “islands” as follows: (1) Combustor Fuel Preparation and SolidsHandling; (2) Fuel Combustion and Media Heating; (3) Fuel CombustionProducts Heat Recovery and Steam Generation; (4) Steam Calcination ofDried Lime Mud; (5) Lime Mud Drying; (6) Carbon Dioxide Recovery; (7)Lime Mud Preparation; and (8) Calcined Lime Mud Cooling and Pelletizing.

Combustor Fuel Preparation and Solids Handling

The primary fuel, wet WWTP sludge, is delivered [1] to an indirectlysteam heated dryer (2) which utilizes low-pressure, saturated steam [3]provided by the mill. Sweep air stream (4) acts to carry the evaporatedwater and prevent condensation. Condensed steam (5) is returned to themill. The dryer exit gas [6] reports to the fabric filter (7) where itis separated into clean gas air/water mixture [8] and captured dryercarryover solids [9].

Dried WWTP sludge, [10] is mixed with dryer carryover solids [9] andjointly fed [11] to silo (12). The combined streams [13] arepneumatically injected [14] into fluid bed combustor lower section, (15)via pressurized air [16] provided by blower (17).

Clean dryer exit gas [8] reports to a direct contact water cooler (18)using mill process water [19] as the coolant. Condensed hot water [20]is returned to the mill's process hot water system. Cooled, saturatedexhaust air [21] is pressurized and delivered via blower (22) to thefluid bed combustor's secondary combustion air inlet stream, [23].

Under normal operating conditions, there is a close balance between WWTPsludge supply and combined calciner and dryer enthalpy load. Shouldthere be a enthalpy supply shortage, dried, precipitated lignins fromthe mill's recovery boiler area can be added directly [24] to silo (12)and jointly injected [14] with dried WWTP sludge into fluid bedcombustor lower section [15] via pressurized air [16] provided by blower(17).

Should the mill not have available precipitated lignins, fuel silos,(25) and (26) provide increasingly higher cost fuels. Silo (25) storeswet biomass, the preferred secondary fuel due to its low cost andavailability in Kraft paper pulp mills. The biomass is delivered via[27] to a chipper/shredder (28) after which it [29] is mechanicallyconveyed (30) to the fluid bed combustor lower section (15) via screwfeeder (31) or other appropriate feeding device.

A silo (26) stores costlier fossil fuels such as high sulfur coal,petroleum coke, shredded tires, waste coal, etc. should biomass beunavailable in sufficient quantities to satisfy the system's totalenthalpy requirements. They are delivered [32] to a pulverizer (33)pulverized into a fine powder [34] and then pneumatically conveyed [35]into the lower fluid bed combustor section (15) using transport air [16]provided by blower (17). The fuel is pulverized by (33) to ensure rapidcombustion and complete ash separation from hot heat transfer media.

The silo (36) contains makeup media to replace that destroyed by cyclichandling when utilizing MMHT. The media can be alumina, silica, mulliteor other solid, inert materials noted for strong thermal cycling andmechanical strength characteristics. It is gravity delivered via [37] tothe cooled media return “J” valve, (38).

Other fuels injected into the fluid bed combustor lower section (15) areall collected Kraft paper pulp mill NCGs and fuel oil or natural gas,[39]. The NCGs provide a noticeable enthalpy input and can be safelydisposed of in an environmentally sound manner during normal NCGincinerator downtime periods. Oil and natural gas are used for rapidtemperature trimming, and startup.

To neutralize emitted SO₂ from solid fuel, NCG, and fuel oil combustion,sorbent limestone is added to the fluid bed combustor lower section (15)at a Ca/S molar ratio between 1.0 and 2.5. This technique is well knownto those familiar with fluid bed combustor design. Market qualitylimestone is stored in silo (40) and conveyed [41] to pulverizer (42)and converted into finely ground limestone [43] prior to pneumaticinjection [44] into the lower fluid bed combustor lower section (15)using transport air [16] provided by blower (17). The limestone ispulverized so as to ensure rapid SO₂ sorption and subsequent completeseparation from hot heat transfer media.

Lastly, separated combustor ash [45] from storage silo (46) ispneumatically conveyed [47] into the fluid bed combustor lower section,(15) by transport air [16] provided by blower (17). This ash may berecycled to ensure complete fuel carbon content combustion. Thistechnique is well known to those familiar with fluid bed combustordesign.

Fuel Combustion and Media Heating

WWTP sludge and precipitated lignins [14], other fuels [30, 35, and 39],ash [47] and limestone [44] are injected into the fluid bed combustorlower section (15) of a refractory-lined circulating fluid bed combustorwhich may be comprised of eight sections, (15), (48), (49), (50), (51),(52) (53) and (54).

Pressurized combustion air, between 1.14 and 1.36 bar (16.5 to 19.7psia) and pre-heated to between 149° C. and 204° C. (300° F. to 400° F.)is introduced via stream [55] into the cylindrical or rectangularcombustor fluidizing air plenum, (54). The amount of air introduced isless than the stoichiometric amount required for full combustion of allfuels entering cylindrical or rectangular combustor section [15] so asto ensure reducing conditions within this combustor section. A gasdistribution grid (nozzle or orifice plate) mechanically separatescombustor sections [15] and [54]. All these techniques are well known tothose familiar with fluid bed combustor design.

Fluid bed combustor lower section (15) is a dense bubbling bed madeprimarily large media particles. These large media particles are sizedto not elutriate when the cross-sectional gas combustion product gasvelocity within fluid bed combustor lower section (15) is 6.1 meters persecond (20 feet per second) or greater. Introduced fuels are gasifiedand partially combusted in this sub-stoichiometric combustion section,their released enthalpy being absorbed by 788° C. to 882° C. (1450° F.to 1620° F.) cooled media [56] comprised of that returned from thecalciner via stream [57], combustor bed return via stream [58], andmakeup media via stream [37].

Returned cool media [56] enters fluid bed combustor lower section [15]via a “J” valve (38, a.k.a. loop-seal) fluidized with super-heated steam[59] at 2.07 bar (30 psia) and 204° C. (400° F.).

As large clinkered ash particles increase in volume in fluid bedcombustor lower section (15), they, and some media are gravitydischarged via a high-temperature “cone” valve (60) and are externallyseparated with large media [58] being returned to fluid bed combustorlower section (15) via cooled media stream [56]. Such valve designs arewell known to those familiar with fluid bed combustor and calcinerdesign techniques.

Pressurized secondary combustion air [61], between 1.15 and 1.22 bar(16.7 to 17.7 psia) and pre-heated to between 149° C. and 204° C. (300°F. to 400° F.) is provided to complete fuel burnout and circulatingmedia heating in cylindrical or rectangular combustor section (48).

This secondary combustion air introduction technique is widely used withcirculating fluid bed boilers. Total excess oxygen exiting fluid bedcombustor transport section (48) is between 10% and 35% above thatrequired for stoichiometric combustion and is dependent on a givencombined fuel mix's combustion characteristics. The combustorequilibrium temperature for ash, media, and gas will be between 843° C.and 927° C. (1550° F. and 1700° F.).

Due to the high superficial gas velocity of combustion products in fluidbed combustor transport section (48), ash particles and circulatingmedia are vertically transported together into cylindrical combustorfreeboard section (49) in excess of the media's transport velocity. Thisvelocity normally does not exceed 6.1 mps (20 fps).

Combustor freeboard section (49) is a cylindrical expanded diameterupper chamber that acts to disengage reheated circulating media atbetween 843° C. and 927° C. (1550° F. and 1700° F.) from ash. Itscross-sectional area is such that the gaseous combustion productsexiting fluid bed combustor transport section (48) are rapidly expandedto a lower velocity. This lower gas velocity is less than thecirculating media particles' vertical transport velocity of 6.1 mps (20fps) but much greater than the fine ash particles' vertical transportvelocity.

In this manner, entrained ash exits combustor freeboard section (49) viastream [62] with fuel combustion products while reheated, largelyash-free media drops by gravity into a combustor storage section (50)that is an integral hot media storage hopper. The volume of combustorstorage section (50) is such that it can store hot media when thecalcination step requires only 25% of the combustor's full enthalpyrelease capacity.

Depending on calciner enthalpy needs, the reheated circulating media iswithdrawn at an appropriate controlled rate from combustor storagesection (50) via multiple discharge ports, the flow through eachdischarge port being externally controlled by multiple high-temperaturecone valves (51). The number of discharge ports and cone valves isbetween 4 (four) and 24 (twenty four), the exact amount a function ofcalciner and dryer enthalpy needs (media rate) and related fluid bedcross-sectional area. Such cone valves are well known to those familiarwith bubbling fluid bed calciner design.

Reheated circulating media discharged from combustor storage section(50) via multiple cone valves (51) may contain some entrained fine ash.This ash if returned to the calciner with reheated circulating media mayeventually contaminate the Kraft mill's recausticizing circuit.Therefore, the reheated media/ash mix first enters a “cone cap andslope” stripper (52) where the ash contaminated media flows downward, bygravity, over a series of cone caps and slopes (see detail “A” on theprocess flow diagram). Steam [63] at 2.07 bar (30 psia), or less, andpre-heated to 204° C. (400° F.) flows upward through the stripper,separating the ash from the downward flowing circulating media. Suchdesigns are well known to those familiar with oil refinery fluidcatalytic cracking design.

The ash/steam mixture [64] is vented into the combustor freeboardsection, (49). Cleaned hot circulating media [65] at 843° C. and 927° C.(1550° F. and 1700° F.) is gravity discharged from the stripper (52) andreports to the fluid bed calcination section [66] via injectors [53]properly prepared to provide the calcination and drying steps netendothermic enthalpy need.

Ash and sulfated and un-sulfated limestone particles enter hot cyclone(67) via [62] where most of the incoming solids are separated from hotgaseous combustion products at 843° C. and 927° C. (1550° F. and 1700°F.). The cyclone solids underflow [68] enters silo (46) where it issplit into two streams. One stream [45] is the previously mentionedsolids recycle flow and the second [69] enters a small water-cooled disccooler (70). Cooled solids [71] less than 93° C. (200° F.) then exitsthe system at this point.

The combustor is pre-heated on initial start-up by ambient temperatureprimary combustion air [72] heated to no greater than 816° C. (1500° F.)via oil or natural gas [73] in a direct-fired heater (74). When thefluidized media bed in fluid bed combustor lower section (15) reaches asuitable autogenic fuel combustion temperature, premium fuel (oil, gas,or coal) is injected [39] directly into the combustor bed to elevate itstemperature to the desired operating temperature. Premium fuel use [39]is gradually disengaged as WWTP sludge, biomass, and/or fossil fuel feedcommence.

Combustion Products Heat Recovery and Steam Generation

Solids and hot combustion products exiting the combustor are separatedin a cyclone with ash exiting the system via a conventional rotarycooler. Largely cleaned, hot combustion products then enter multipleconvective heat exchangers that, in counter-current series; superheatlow pressure mill steam for calciner fluidization and internalcalcination process service needs; generate high pressure superheatedsteam for export to the mill's steam loop, and preheated combustion air.Cooled combustion products exit to ambient via conventional gas clean-updevices.

Hot, ash laden combustion products [62] exiting combustor freeboardsection (49) and entering cyclone (67) may be mixed with ammonia or urea[75] to reduce nitrogen oxide emissions with selective non-catalyticremoval (SNCR) technology known to those familiar with fluidized bedboiler design. Should the nitrogen oxide content exiting combustorfreeboard section (49) be less than that required by law, then this stepis not required.

Hot, largely ash-free combustion products exiting cyclone (67) via [76]may be mixed with natural gas or oil [77] in incinerator (78) toincrease the temperature to 982° C. (1800° F.) at a sufficient residencetime in incinerator (78) such that any chlorinated organics (dioxins)can be destroyed. Some chlorinated organics may, or may not, be presentin the WWTP sludge fuel and could transit the combustor without beingdestroyed. This technique is well known to those familiar with wastefuel combustor design. Should the dioxin content exiting combustorfreeboard section (49) be less than that required by law, then this stepwill not be necessary.

Hot gases [79] exiting incinerator (78) enter gas/gas heat exchangersection (80) superheated low pressure saturated steam [81] from the millat 2.07 bar (30 psia) to 538° C. (1000° F.). This super-heated lowpressure steam is then largely directed into two (2) flows; most of itreports to the calciner inlet fluidizing gas plenum (82) via [83]. Asmaller amount reports to the calcine cooler inlet fluidizing gas plenum(84) via [85].

The remaining super-heated low pressure steam is tempered with boilerfeed-water [86] to 204° C. (400° F.) and directed into six (6) flows;via [87] to the calciner hot media injection valve, (53); via [59] tothe combustor cool media injection valve, (38); via [63] to thecombustor hot media stripper, (52); via [88] to the calciner cool mediareturn stripper, (89); via [90] to the dry lime mud injection line,[91], and via [92] to the calcined lime mud re-injection line, [93].Streams [90] [91] and [92] are all located on both FIGS. 4 and 5.

Cooled combustion products exit gas/gas heat exchanger section (80) andenter a second gas/gas heat exchanger section super-heater section (94)which superheats saturated high pressure steam [95] exiting steam drum(96) at between 104.5 bar (1515 psia) and 42.4 bar (615 psia). Thesuper-heated high pressure steam temperature will be between 538° C.(1000° F.) and 399° C. (750° F.) when exported to the Kraft pulp andpaper mill's main steam loop via [97].

Cooled combustion products exit super-heater section (94) via stream[98] and enter forced-circulation boiler economizer/evaporator section(99). The steam/water mix [100] generated in economizer/evaporatorsection (99) enters steam drum (96) where saturated high pressure steambetween 104.5 bar (1515 psia) and 42.4 bar (615 psia) exits via [95] andreports to super-heater section (94). Steam drum (96) saturated liquidunderflow [101] is extracted and boosted to evaporation pressure byboiler circulation pump (102) being delivered via [103] toeconomizer/evaporator section (99). Pre-heated boiler feed-water,originally from the mill's boiler-house, enters steam drum (96) via[104] having been pre-heated in calcine cooler section (105). Stream[104] is depicted on FIGS. 4 and 5.

Still further cooled combustion products leaving economizer/evaporatorsection (99) via [106] are split into two streams, one entering primaryair pre-heater section (107) and the second entering secondary airpre-heater section (108).

Primary air pre-heater section (107) pre-heats primary combustion airdelivered via [109] by primary combustion air blower (110) at between1.15 bar and 1.36 bar (16.7 psia to 19.7 psia). The primary combustionair exiting air pre-heater section (107) via [55] is heated to between149° C. and 204° C. (300° F. to 400° F.) and then reports to combustorfluidizing air plenum (54).

Secondary air pre-heater section (108) pre-heats secondary combustionair delivered via [23] by secondary combustion air blower (111) atbetween 1.15 bar and 1.22 bar (16.7 psia to 17.7 psia). The secondarycombustion air exiting (108) via [61] is heated to between 149° C. and204° C. (300° F. to 400° F.) and then reports to fluid bed combustortransport section (48).

Finally, completely cooled combustion products exiting primary airpre-heater section (107) and secondary air pre-heater section (108) via[112] enter a fabric filter bag-house (113) where residual fine fuel ashand sulfated and unsulfated limestone are separated from combustionproducts. Ash [114] exits the system and ash-free combustion productsare discharged to atmosphere via [115] by exhaust fan (116).

Steam Calcination of Dried Lime Mud

CO₂ is liberated in a cylindrical steam fluidized “bubbling fluid bed”(BFB) calciner using inert hot media entering the calciner at a highertemperature than the calciner's operating bed temperature. Entering hotmedia releases its stored enthalpy as the endothermic heat load requiredto calcine dried lime mud to calcined lime mud product at the properreaction conditions. Calciner fluidization steam enters at a controlledamount to insure that the gaseous CO₂-steam reaction atmosphere iscontinually maintained at a CO₂ partial pressure adequate to drive thecalcination reaction. The CO₂-steam reaction atmosphere exiting thecalciner is directed to a dryer to evaporate water associated with limemud provided by the Kraft pulp and paper mill and has the properenthalpy requirement to dry incoming lime mud at the lowest possibledryer outlet temperature and maximum relative water saturation.

Gravity delivered to the fluid bed calciner feed injectors (53) is hot,stripped media [65] at a rate dependent on the calciner's and dryer'sendothermic enthalpy requirement. The hot media is motivated through theinjectors by super-heated low pressure steam [87] delivered to theinjector's hot media entry point at 204° C. (400° F.). The injector ispreferably an “L” valve design, but may also be of the “J” type. Suchvalve designs are well known to those familiar with fluidized bed designtechniques.

Dried lime mud is transported [91] by a pressurized CO₂/super-heatedsteam mixture to the calciner media injector, (53) at a point downstreamof the steam/hot media mixing point. Blower (117) (FIG. 4) receivesexport quality CO₂ [118] (FIG. 4) from the CO₂ product area (outside ofthe invention's battery limits) and pressurizes it to ensure that theinjection pressure into the cylindrical calciner bed is not less than1.57 bar (22.7 psia). Superheated steam is injected into [91] via [90].

The resulting steam/hot media/dry lime mud/CO₂ mixture is injected bycalciner media injector (53) at a pressure not less than 1.57 bar (22.7psia) into the base of cylindrical fluid bed calcination section (66) ata point directly above the calciner's circular gas distribution plate.The total number of injectors may vary from between four (4) andtwenty-four (24) dependent on calcined lime mud production capacity andcalciner distribution plate cross-sectional area.

Calcined lime mud in stream [93] is injected into fluid bed calcinationsection (66) at a pressure no less than 1.57 bar (22.7 psia) by millsteam [92] (FIG. 4) at 2.07 bar (30 psia), and 204° C. (400° F.).Calcined lime mud recycling ensures complete lime mud calcination whileproviding extra nucleation sites to mitigate sodium salt fouling in thecalciner fluid bed. The amount of calcined lime mud entering via [93] isbetween 15% and 25% of the total calcined lime mud production ratedependent on actual need.

Calciner fluidization steam at 538° C. (1000° F.) is delivered [83] intothe calciner fluidizing gas plenum (82). The steam pressure is notgreater than 2.07 bar (30 psia) but at a pressure adequate to fluidizethe bubbling media bed in fluid bed calcination section (66) at afluidized bed height of no greater than 2.44 meters (8.0 feet). Thesteam amount entering calciner fluidizing gas plenum (82) will becontrolled so as to ensure the CO₂ partial pressure in fluid bedcalcination section (66) is no greater than 90% of the CO₂ equilibriumpartial pressure at fluid bed calcination section (66)'s maximumfluidized bed pressure and minimum fluidized bed temperature when alsoconsidering steam/CO₂ mixtures contained in calciner input streams [59],[91], [87] and [93]. Lastly, the total enthalpy, when consideringtemperature and mass amounts, contained in all gases exiting in stream[119] will be sufficient to satisfy the lime mud drying step'srequirement.

Fluid bed calcination section (66) is fluidized at the distributor platetop by steam at a velocity greater than the circulating media'sincipient fluidization velocity but less than its maximum transportvelocity of 6.1 mps (20 fps) and always greater than the largestcalcined lime mud particle's transport velocity.

Fluid bed calcination section (66) expands in cylindricalcross-sectional area as CO₂ is liberated by the calcination reaction andstops expanding at calciner freeboard section (120) entry point. Thisincreasing cross-sectional area insures that the CO₂/steam gas mixtureexits fluid bed calcination section (66) and enters calciner freeboardsection (120) at a velocity greater than the media's incipientfluidization velocity but less than its minimum transport velocity butalways greater than the largest calcined lime mud particle's transportvelocity. This ensures that elutriated calcined lime mud is transportedinto calciner freeboard section (120) and media is disengaged from thecalcined lime mud, falls back, and returns to fluid bed calcinationsection (66).

Cooled media exits the calciner through a gravity discharge overflowport located at the interface of fluid bed calcination section (66) andcalciner freeboard section (120), i.e., the top surface of the calcinerbubbling fluid bed. Media discharged from fluid bed calcination section(66) may contain some entrained calcined lime mud. This calcined limemud, if returned to the combustor with the media will create an economicloss. Therefore, the media/calcined lime mud mix enters a “cone cap andslope” stripper (89) where the media/calcined lime mud mix flowsdownward, by gravity, over a series of cone caps and slopes (see detail“A” on the process flow diagram). Steam [88] at 2.07 bar (30 psia), orless, and pre-heated to 204° C. (400° F.) flows upward through thestripper, separating calcined lime mud from the downward flowing media.Such stripper designs are well known to those familiar with refineryfluid catalytic cracking design techniques. The stripped calcined limemud and associated steam enter calciner freeboard section (120).

Cool, stripped media exits “cone cap and slope” stripper (89) viadischarge stream [57], and is combined with make-up media [37] andrecovered media [58] with all reporting to the media return valve (38).Valve (38) returns media to fluid bed combustor lower section (15) aspreviously explained. Valve (38) is preferably a “J” type valve but mayalso be an “L” type valve. Such valve designs are well known to thosefamiliar with fluidized bed design techniques.

The number of strippers (89) and valves (38) will be not less than two(2) to ensure proper distribution of returned media into the fluid bedcombustor lower section (15).

Calcined lime mud particles elutriated into the calciner freeboardsection (120) represent the total calcined lime mud production rate.This calcined lime mud, along with the exiting CO₂/steam gas mixture,exits calciner freeboard section (120) via stream [119] prior toentering hot cyclone (121).

The calcined lime mud particle temperature in both fluid bed calcinationsection (66) and calciner freeboard section (120) will be not less than760° C. (1400° F.) and not greater than 854° C. (1570° F.) and typicallybetween 791° C. (1455° F.) and 800° C. (1472° F.). Hot media enteringfluid bed calcination section (66) via injectors (53) will be not lessthan 832° C. (1530° F.) and not greater than 927° C. (1700° F.). Cooledmedia entering media stripper (89) will typically be 28° C. (50° F.)higher than the exiting calcined lime mud particle entering calcinerfreeboard section (120). This will ensure rapid heat transfer betweenthe incoming hot media and the incoming dried lime mud.

The entire cylindrical calciner vessel represented by sections (82),(66), and (120) will surround combustor cylindrical, or rectangular,fluid bed combustor transport section (48). The combustor storagesection (50) will share a common floor/roof with calciner freeboardsection (120). In this manner the calciner and combustor are integratedinto a compact, vertical design to minimize land area requirements. Allinterior surfaces of the calciner and combustor will be refractory linedfor abrasion resistance and thermal insulation purposes. Such designsare well known to those familiar with fluidized bed design techniques.

Lime Mud Drying

There are two lime mud drying routes that can be utilized. The use ofeither drying route is primarily a function of desired CO₂ productquality, initial lime mud moisture content, and overall fuel cost. Aspray dryer can be used when CO₂ quality in excess of 95% purity isrequired and when the lime mud moisture content received from the millis 30% or more, although higher solids content lime muds, when waterdiluted, are acceptable. This drying route consumes the highest amountof fuel fed to the combustor. A flash dryer can be used when CO₂ qualityin excess of 90% is required and/or with lime mud moisture contents lessthan 30% up to the maximum solids content produced by the lime mudfiltration system. This drying route consumes the lowest amount of fuel.The spray dryer route is depicted on FIG. 4 and the flash dryer route onFIG. 5.

Spray Dryer Route

In accordance with FIG. 3, the hot CO₂, steam, and elutriated calcinedlime mud product mixture exiting calciner freeboard section (120) via[119] is largely cleaned of calcined lime mud in a hot cyclone (121).Separated larger particle calcined lime mud, representing most of thecalcined lime mud production rate, exits cyclone (121) via [122] whereit enters the first stage of the first fluid bed cooler section, (123).

The hot CO₂/steam gas and very fine particle calcined lime mud mixtureexits cyclone (121) via [124] and reports to the spray dryer enteringvia vertical “air disperser” duct (125) that rises through spray dryerconical chamber (126) and cylindrical chamber (127) terminating at apoint beneath the rotary disc atomizer, (128). Hot drying gases and fineparticle calcined lime mud exit air disperser duct (125) via a vaned, ornon-vaned, opening, a.k.a. a “chimney air disperser”.

The rotary atomizer (128) utilizes an abrasion-resistant spinning discto atomize the lime mud slurry [129] into very fine droplets. Thisslurry is a pumpable lime mud/water mixture at no less than 65% andtypically 70% total solids content. The resultant fine droplet cloud isimmediately contacted by the dispersed entering hot gases [124] exitingair disperser duct (125) and is instantaneously converted into fine dryparticles and vaporized water.

Both the air disperser duct and atomizer designs referred to are thoseoffered by GEA Niro, Copenhagen, Denmark. Such techniques are well knownto those familiar with designers of high tonnage spray drying systemsfor minerals processing.

The rapid evaporation process immediately quenches gases in thecylindrical chamber (127), and conical chamber (126), to a temperatureno less than 91° C. (195° F.) and no greater than 104° C. (220° F.)dependent on the entering gas temperature and composition, lime mudslurry [129] total solids content, and the need to maintain a dryparticle moisture content at no greater than 2% by weight. At theseconditions most, if not all, of the NaOH present in the lime mud slurrywill be converted to Na₂CO₃ due to the CO₂ content and temperature inthe drying chamber.

A very small portion of the CaO contained in the calcined lime mudentering the dryer with the hot gases via [124] is also rehydrated andrecarbonated to Ca(OH)₂ and CaCO₃ due to the CO₂/water vapor mixturepresent in the drying chamber. These solids are mixed with dry lime mudreporting to the calciner where it is re-calcined.

The coarse particle size dry lime mud portion is captured in conicalchamber (126) and gravity discharged via a rotary valve through exitduct [130] and reports to the dry lime mud storage silo, (131). Thisseparation technique is well known to those familiar with designers ofhigh tonnage spray drying systems for minerals processing.

Remaining dry lime mud, representing finer particle sizes, exit conicalchamber (126) with the cooled CO₂/water vapor mixture via [132] andreport to the dryer cyclone (133) which separates larger dry lime mudparticles that then exit the cyclone base via a rotary valve and duct[134] and report to dry lime mud storage silo (131).

Gases [135] exiting dryer cyclone (133), containing the finest residualdry lime mud particles, enter a fabric bag-house filter, orelectrostatic precipitator (ESP), (136). These separated dry lime mudparticles exit (136) via a rotary valve and duct [137] and report to thedry lime mud storage silo, (131).

The collected spray dried lime mud in silo (131) is discharged from itsconical silo base through a multiplicity of discharge valves numberingno less than four (4) and no greater than twenty four (24) but always anumber equivalent to the number of calciner hot media/dried lime mudinjectors, (53). Blower (117) receives CO₂ vapor from the CO₂ productstorage area (beyond this invention's battery limits) and boosts it to apressure satisfactory to transport the dry lime mud that exits dry limemud storage silo (131) to injectors (53) to ensure that the injectordischarge pressure entering calciner section (66) is no less than 1.57bar (22.7 psia).

Dry lime mud free gases exiting (136) are extracted by exhaust gas fan(138) that then deliver this CO₂/water vapor mixture [139] gas to aconventional direct contact water scrubber that largely separates thewater vapor from all entering gases. Fan (138), and stream [139], arelocated on both FIGS. 4 and 5.

Flash Dryer Route

Unless otherwise noted, all stream numbers are depicted on FIG. 5. Inaccordance with FIG. 3, The hot CO₂, steam, and elutriated calcined limemud product mixture exiting the calciner via [119] is largely cleaned ofcalcined lime mud in cyclone (121). Separated larger particle calcinedlime mud, representing most of the calcined lime mud production rate,exits cyclone (121) via [122] where it enters first fluid bed coolersection, (123) which is located on both FIGS. 4 and 5.

Exiting cyclone (121) via [124] is a hot CO₂/steam and residual fineparticle calcined lime mud mixture at 760° C. (1400° F.) to 854° C.(1570° F.). Intercepting stream [124] is stream [183] that is apressurized dense phase mixture of CO₂ and cooled calcined lime mudhaving first been discharged from fluid bed calcined lime mud coolersection (180) via [179] at approximately 93° C. to 121° C. (200° F. to250° F.) with blower (198) providing injection/transport CO₂ via stream[197] from the CO₂ product area (outside of the invention's batterylimits). This stream interception causes stream [124] to be tempered(“quenched”) to no greater than 593° C. (1100° F.) in the temperingcyclone, (184). Lowering the temperature ensures that flash dryer (185)effectively operates without mechanical or process problems.

Separated, and reheated, calcined lime mud at a temperature somewhatless than 593° C. (1100° F.), exits tempering cyclone (184) via [186]and enters the first fluid bed cooler section, (123).

The cooled CO₂/steam and fine particle calcined lime mud mixture exitsthe tempering cyclone (184) at no greater than 593° C. (1100° F.) via astream [187]. Intercepting stream [187] is emergency quench CO₂ [188]from the CO₂ product area (outside of the invention's battery limits)and, if needed, emergency quench water [189] to ensure that the hotgases and residual fine particle calcined lime mud entering flash dryer(185) do not exceed 593° C. (1100° F.).

Prepared lime mud [190] exiting the pug-mill feeder (191) enters theflash dryer (185) at a point above the gas stream [187] entry point.This lime mud is dispersed into gas stream [187] by appropriate means toensure that rapid water evaporation in the flash dryer (185) occurs at atemperature no less than 96° C. (205° F.) and no greater than 104° C.(220° F.) dependent on the entering gas temperature, total lime mudsolids content, and the need to maintain a dried particle moisturecontent at no greater than 2% by weight. At these conditions most, ifnot all, of the NaOH present in the entering lime mud is converted toNa₂CO₃ due to the CO₂ content and temperature in the flash dryer (185).

A very small portion of the CaO contained in the calcined lime mudentering flash dryer (185) via [124] is rehydrated and recarbonated toCa(OH)₂ and CaCO₃ due to the CO₂/water vapor mixture present in thedryer. These solid are commingled with dry lime mud which eventuallyreports to the calciner where it is recalcined.

The dry lime mud and cooled CO₂/water vapor mixture exiting the flashdryer (185) via exit duct [132] reports to dryer cyclone (133). Dryercyclone (133) separates entering larger dry lime mud particles that thenexit the cyclone base via a rotary valve and duct [134] and then reportto the dry lime mud storage silo, (131).

Gases exiting dryer cyclone (133), which contain the residual finest drylime mud particles, enter a fabric bag-house filter, or electrostaticprecipitator (ESP), (136) via [135]. The separated dry lime mudparticles exit bag-house filter or ESP (136) via a rotary valve and[137] and then report to the dry lime mud storage silo, (131).

The collected dry lime mud in dry lime mud storage silo (131) isdischarged from its conical silo base through a multiplicity ofdischarge valves (between 4 and 24 in number) but always a numberequivalent to the number of calciner hot media/dried lime mud injectors,(53). A blower (117) receives CO₂ vapor from the CO₂ product storagearea (beyond this invention's battery limits) via [118] and boosts it toa pressure satisfactory to transport solids exiting dried lime mudstorage silo (131) to hot media/dried lime mud injectors (53) to ensurean injector discharge pressure entering the fluid bed calcinationsection (66) is no less than 1.57 bar (22.7 psia).

Dry lime mud free gases exiting bag-house filter or ESP (136) areextracted by an exhaust gas fan (138) that delivers this CO₂/water vapormixture gas to a conventional direct contact water scrubber that largelyseparates the water vapor from all entering gases.

Carbon Dioxide Recovery

The CO₂ recovery process island description is the same for either thespray dryer or flash dryer route and is depicted on both FIGS. 4 and 5.The cleaned CO₂/water vapor mixture leaves the bag-house filter or ESP(136) via an exhaust fan (138) and reports to the direct contact waterscrubber lower contact stage (141) via [139]. This direct contact waterscrubber uses mill cooling tower water [140] entering at approximately38° C. (100° F.) and fresh mill water (142) at approx. 10° C. (50° F.)in its upper contact stage (143). This type design is well known tothose familiar with the practice of purifying industrial gas streams.

The counter-current direct contact between cooling water and incominghot gas further removes residual lime mud particles and condenses watervapor to concentrate CO₂ in the water scrubber's exhaust gas stream[144] to a saturation temperature of approximately 16° C. (60° F.). Thislow temperature, water-saturated CO₂ gas stream reports to a facility(beyond this invention's battery limits) designed to remove residualgaseous/solid impurities and create a marketable liquid or gaseous CO₂product. This technology is well known to those in the industrial gasindustry.

Rejected scrubber water and small amounts of hydrated calcined lime mud[145] is collected in the direct contact scrubber sump at approximately82° C. (180° F.) and reports to a collection tank, (146). The vent gas[147] from (146) contains some CO₂ that is directed to the directcontact scrubber exhaust gas stream [144].

For both the spray and flash dryer routes the collected, rejectedcalcined lime mud free hot water in collection tank (146) then flows tothe mill's recausticizing circuit via [148] for dilution and washingpurposes. This is as depicted in FIG. 5. For the spray dryer route, hotwater with hydrated calcined lime mud underflow [149] reports to tank(150).

Lime Mud Preparation

The lime mud preparation island is different for each drying route andis depicted on FIG. 4 for the spray dryer route, and FIG. 5 for theflash dryer route. The spray dryer route requires a liquid, pumpablefeedstock whereas the flash dryer route can accept a relatively dryerfilter cake produced in the manufacturing operation.

Spray Dryer Route

Unless otherwise noted, all stream numbers are depicted on FIG. 4.Pumpable lime mud slurry for the spray dryer atomizer is prepared inopen top stirred tank (150) when lime mud filter cake [151] at up to 90%solids content is fed via screw feeder (152) and mixed with hot waterand residual hydrated calcined lime mud reporting from the directcontact scrubber via [149]. The total solids content in open top stirredtank (150) is controlled to no less than 65% but typically 70% to ensureeasy pumping and atomization. The actual lime mud slurry water contentwill be a function of a specific mill's recausticizing circuitoperations.

A sufficient amount of liquid H₂O₂ or gaseous O₂ [153] is also added toopen top stirred tank (150) to fully oxidize trace, residual amounts ofNa₂S in the stirred slurry into Na₂SO₄. This conversion preventscontaminating CO₂ product gas [144] with gaseous total reduced sulfur(TRS) compounds that could be generated in the spray dryer or calciner.This also helps to mitigate scaling and fouling in the calciner byaltering the Na₂CO₃/Na₂SO₄ mass ratio into a higher melting pointregion.

A small amount of Na₂SO₄ or Na₂CO₃ solution [154] may also be added tomitigate scaling and fouling in the calciner by altering theNa₂CO_(3/)Na₂SO₄ mass ratio into a higher melting point region.

There are multiple open top stirred tanks (150) to ensure consistentadjustment of total solids content and sodium salt composition andcontent prior to transfer [155] to the enclosed stirred feed tank, (156)via the pump (157). Final corrective amounts of liquid H₂O₂ or gaseousO₂ may be added via [158] into pump (157).

The enclosed stirred feed tank, (156) receives prepared lime mud slurryfrom open top stirred tank (150) via [155]. Sweep steam [159] is addedto the enclosed stirred feed tank (156) freeboard to remove anyentrained residual air vented to ambient via vent [160]. The preparedlime mud slurry is then pumped via stream [129] by a positivedisplacement pump (161) to the spray dryer rotary atomizer (128) at avariable, but controlled rate considering the product moisture contentand the drying gas composition and temperature.

Flash Dryer Route

Unless otherwise noted, all stream numbers are depicted on FIG. 5. Flashdryer (185) lime mud feed is prepared in a pug mill feeder (191) whenlime mud filter cake [192] at up to 90% total solids content is mixedwith a sufficient amount of liquid H₂O₂ [193] to oxidize trace, residualamounts of Na₂S in the lime mud into Na₂SO₄. This conversion preventscontaminating the CO₂ product gas [144] with gaseous total reducedsulfur (TRS) compounds that could be generated in the flash dryer orcalciner and helps to mitigate scaling and fouling in the calciner byaltering the Na₂CO_(3/)Na₂SO₄ mass ratio into a higher melting pointregion.

A small amount of Na₂SO₄ or Na₂CO₃ solution [194] may also be added tomitigate scaling and fouling in the calciner by altering theNa₂CO_(3/)Na₂SO₄ mass ratio into a higher melting point region.

The total solids content exiting pug mill feeder (191) via stream [190]is slightly reduced from the maximum 90% total solids present in limemud filter cake [192] with the addition of liquid H₂O₂ [193] andNa₂CO_(3/)Na₂SO₄ solution [194]. This exiting lime mud solids content instream [190] should be maintained at no less than 85% to ensure properfeeding and dispersion into the flash dryer (185) and is accomplished byadding dry lime mud [195] to pug mill feeder (191) that has beenre-cycled from dry lime mud storage silo (131) via mechanical means.

The lime mud feed rate to the flash dryer (185) from pug mill feeder(191) is controlled to maintain a constant dried lime mud productmoisture content considering the interrelationship of the actual flashdryer inlet drying gas composition and temperature. Surge dry solids arestored in dry lime mud silo (131). This control technique is well knownto those familiar with the practice of industrial flash drying.

Calcine Cooling and Pelletizing

The calcined lime mud cooling and pelletization island description isessentially the same for both drying routes as depicted on FIGS. 4 and5, except for several variations, which are depicted in FIG. 5 for theflash dryer route. Hot calcined lime mud [122] discharged from cyclone(121) and reheated calcined lime mud [186] discharged from the temperingcyclone (184) (when using the flash dryer route) report to a rectangularfluid bed cooler section (123) via a dip-leg seal. First fluid bedcooler section (123) is fluidized by steam [85] entering via first fluidbed cooler plenum section (84) at 2.07 bar (30 psia) and 538° C. (1100°F.) to ensure a proper seal between the cyclone dip-leg and ambient.Therefore, any gas drawn up into the negative pressure cyclones (121)and (184) is steam which will not contaminate the CO₂ recovery loop.

The first fluid bed cooler section (123) is a “back-mix” fluidized beddesign to ensure uniform mixing of fluidization steam and entering hotcalcined lime mud. The equilibrium temperature in the first fluid bedcooler section (123) is no less than 600° C. (1112° F.) to ensure thatno calcined lime mud re-hydration occurs. The calcined lime mud exitsfirst fluid bed cooler section (123) by flowing over a full bed widthdivider wall and enters second fluid bed cooler section (105).Fluidization steam exits through the same divider wall opening, drawn bythe negative draft created by cooler exhaust fan (162).

A portion of hot calcined lime mud exits first fluid bed cooler section(123) via an overflow weir into a discharge port [163] at a controlledrate via a cone valve. The amount of calcined lime mud entering [163] isbetween 15% and 25% of the total calcined lime mud production rate andis injected [93] into fluid bed calcination section (66) to ensurecomplete lime mud calcination while also mitigating sodium salt foulingaffects in the calciner fluid bed.

Calcined lime mud entering the second fluid bed cooler section (105) isfluidized by ambient air [165] delivered by a blower (166) throughsecond fluid bed cooler plenum section (167). The fluidized calcinedlime mud flows down the rectangular bed around immersed tube bundleswithin the fluidized bed. Flowing through this tube bundle, counter-flowto the solids flow, is pressurized mill boiler feed-water entering via[168]. Boiler feed-water, now heated to a higher temperature, exits viastream [104] and reports to the inlet of the steam drum (96). Excessheated boiler feed-water exits via stream [169] and reports to themill's boiler-house. Cooled calcined lime mud exits second fluid bedcooler section (105) by flowing over a full bed width divider wallbefore entering third fluid bed cooler section (164).

Calcined lime mud entering third fluid bed cooler section (164) isfluidized by ambient air [165] delivered by a blower (166) through thirdfluid bed cooler section plenum (170). The fluidized calcined lime mudflows along the rectangular fluid bed path around immersed tube bundleswithin the fluidized bed. Fresh mill water [171] flows through this tubebundle, counter-flow to the solids flow, boosted in pressure by a pump(172). Hot water exits via stream [173] and reports to the mill hotwater system or recausticizing circuit for dilution and washing.

Instead of counter-flow pipe bundles, cross-flow plates or pipe bundlesmay also be utilized in fluid bed cooler sections (105) and (164), oranother indirect heat transfer device known to those familiar with thistype fluid bed cooler.

The steam/air mixture in the fluid bed cooler freeboard is withdrawn byan exhaust fan (162) via stream [174]. This exhaust stream reports to afabric filter baghouse (175) and then exhausts to ambient via stream[176]. Collected fine particulate calcined lime mud [178] reports withthe bulk cooler calcine flow [179] as stream [180] to the inlet of thepelletizer (181).

The cooled calcined lime mud, at approximately 93° C. to 121° C. (200°F. to 250° F.), exits the third fluid bed cooler section (164) via anoverflow weir (180) with the rate controlled by a rotary valve. Theproduct calcine is highly reactive since the low-temperature calcinationprocess creates a “soft-burned” product. Its fine particle size alsomakes it dusty. Therefore the calcined lime mud is pelletized inpelletizer (181) to mitigate potential handling and safety problemsbefore being transported via [182] to the mill recausticizing circuit'sslakers.

As depicted on FIG. 5, with the flash dryer route a portion of thiscooled calcined lime mud [179] is extracted at a controlled rate [196]by a rotary valve and is pneumatically transported via CO₂ [183] to thetempering cyclone (184) inlet gas line [124]. This CO₂ is obtained fromthe CO₂ preparation area (outside of this process) via stream [197] andis boosted in pressure by blower (198).

While the present invention has been exemplified by specificembodiments, it will be understood in view of the present disclosurethat numerous variations upon the invention are now enabled to thoseskilled in the art. Accordingly, the invention is to be broadlyconstrued and limited only by the scope of the present disclosure.

1. A method for recovering carbon dioxide comprising: (a) providing wetlime mud sufficiently near a bubbling fluid bed calciner and a spraydryer or flash dryer such that the calciner and flash dryer or spraydryer operate in counter/current gas/solids flow wherein exitingcalciner gases substantially dry the wet lime mud and the resulting drylime mud is fed to the calciner; (b) feeding substantially dry lime mudto the fluid bed calciner wherein the fluid bed calciner is thermallylinked by moving media heat transfer (MMHT) to a circulating fluid bedcombustor by a heat transfer media wherein said media moves between thecalciner and the combustor wherein MMHT provides heat input forcalcination; (c) recycling the heat transfer media from the calciner tothe combustor; (d) recovering excess energy from the process of c)generating superheated high pressure steam; and (e) recovering carbondioxide and calcined lime mud from the fluid bed calciner.
 2. The methodaccording to claim 1 further comprising (f) exporting the superheatedhigh pressure steam to a manufacturing process.
 3. The method accordingto claim 1 further comprising (g) removing substantially all ash fromthe heat transfer media.
 4. The method according to claim 1 wherein thecombustor receives one or more fuels selected from the group consistingof Kraft pulp and paper mill sludge, biomass, precipitated lignins andnon-condensable gases (NCGs).
 5. (canceled)
 6. The method according toclaim 1 wherein the carbon dioxide recovered is of 90% to +99% purity.7. (canceled)
 8. (canceled)
 9. (canceled)
 10. (canceled)
 11. (canceled)12. (canceled)
 13. (canceled)
 14. A method according to claim 1 furthercomprising preheating air entering the combustor and calciner fluidizingsteam by heat exchange with hot combustion products.
 15. A methodaccording to claim 1 further comprising recovering heat as hot processwater or boiler feed water from the hot calcined lime mud by heatexchange in an indirectly cooled fluid bed heat exchanger.
 16. A methodaccording to claim 1 further comprising mixing the-wet lime mud feedwith at least one selected from the group consisting of water, H₂O₂, O₂,Na₂CO₃, and Na₂SO₄.
 17. A method according to claim 1 further comprisinginjecting dry lime mud into the fluidized media bed of the bubblingfluid bed calciner.
 18. A method according to claim 3, wherein (g)removing substantially all the ash from the heat transfer media isperformed in a combustor freeboard section having an expanded diameterby decreasing velocity of the heat transfer media.
 19. A methodaccording to claim 3, wherein (g) removing substantially all the ashfrom the heat transfer media is performed at a temperature of between1550° F. and 1700° F.
 20. A method according to claim 3, wherein (g)removing substantially all the ash from the heat transfer media isperformed by introducing steam into a cone cap and slope stripper. 21.(canceled)
 22. A method according to claim 1, wherein the moving mediamoves between the calciner and the combustor at 1530° F. to 1700° F. 23.A method according to claim 1, wherein the combustor operates at 1550°F. to 1700° F.
 24. A method according to claim 1, wherein the calcineroperates at 1400° F. to 1570° F.
 25. A method according to claim 1,wherein the superheated high pressure steam generated has a temperatureof 750° F. to 1000° F. at a pressure of 615 psia to 1515 psia.
 26. Amethod for calcining calcium carbonate lime mud and converting it tocarbon dioxide and calcined lime mud comprising: (a) providing wet limemud sufficiently near a bubbling fluid bed calciner and a spray dryer orflash dryer such that the calciner and flash dryer or spray dryeroperate in counter/current gas/solids flow wherein exiting calcinergases substantially dry the wet lime mud and the resulting dry lime mudis fed to the calciner; (b) feeding substantially dry lime mud to thefluid bed calciner wherein the fluid bed calciner is thermally linked bymoving media heat transfer (MMHT) to a circulating fluid bed combustorby a heat transfer media wherein said media moves between said calcinerand said combustor wherein MMHT provides heat input for calcination; (c)removing substantially all ash from the heat transfer media; (d)recycling the heat transfer media from said calciner to said combustorwherein said combustor receives one or more fuels and producingcombustion products; (e) recovering excess energy from the process of d)generating superheated high pressure steam; (f) recovering carbondioxide of 90% to +99% purity and calcined lime mud from the fluid bedcalciner; and (g) exporting the superheated high pressure steam to amanufacturing process.
 27. A system for recovering carbon dioxidecomprising a bubbling fluid bed calciner, a circulating fluid bedcombustor, an apparatus or system adapted to thermally link the bubblingfluid bed calciner and the circulating fluid bed combustor, and a dryerselected from the group consisting of a spray dryer and a flash dryer.28. A system according to claim 27 wherein the dryer is a spray dryer.29. A system according to claim 27 wherein the dryer is a flash dryer.